What is this arthropod?

What is this arthropod?

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It is approx 2cm long and was discovered in house after a bush walk & BBQ at Riverside Garden Reserve in Mandurah, Western Australia. It has 6 legs.

Thanks! :-)

This is a Sawfly larvae, and is most likely of the species Perga dorsalis or Perga affinis. The larvae you depict appears to be quite young, but as it matures it'll gain a much darker color, and, depending on whether it's P. affinis vs. P. dorsalis, the abdomen will grow fairly long.

I imagine the developmental progress of your larvae is somewhere between the two following larvae.

A few more images:

Understanding Evolution - The Arthropod Story

2. Sheer Numbers …
a) How many insects exist for every human? ________
b) If all the ants on the earth formed a chain, how many times could the earth be circled? ____
c) What are the most abundant animals in the ocean? _______________

3. Habitat and Distribution
a) What arthropod lives more than a mile below the ocean? ___________
b) b) Where does the largest centipede species live? _________________
c) What arthropod lives at high altitudes? _____________________
d) What arthropod can withstand heats of over 140 º? ________________

4. Ecological Niches
a) What type of arthropod is a farmer? ______________________________
b) Where does the parasitic arthropod Cymothoa exigua live? _____________
c) What type of arthropod cleans fish scales of parasites? ________________

5. Arthropods: A Success Story: Name one reason why arthropods are considered successful.

6. Inherited Characteristics: List the five inherited characteristic of arthropods.

7. a) Describe bilateral symmetry. ________________________________
b) Check those that are bilaterally symmetrical. ___ millipede ____jellyfish _____scorpion

8. a) Segments are grouped into larger sets, such as the abdomen and the ______________
b) Check those that are segmented. ___onychophoran ____ mouse ___jellyfish ____moth

9. a) What substance is the exoskeleton made from? _________________________
b) Check those with an exoskeleton ____ mouse _____moth _____ jellyfish _____millipede

10. a) Side Trip: Support Systems . Name the three types of support systems found in the animal kingdom. _________________________________________________________
b) Which one is possessed by humans? __________________________________
c) What kind of skeleton does an earthworm have? __________________________

11. Jointed Legs
a) What does the word "arthropod" mean? ____________________________________
b) Check those that have jointed appendages.
____millipede ___ moth ____ onychophoran ___ mouse ____ jellyfish

12. a) Name five ways in which limbs can be specialized.

b) Which have many pairs of limbs? ____ mouse ___ moth ____ millipede_____scorpion

13. Based on these data, check all of the following animals that are arthropods.

scorpion moth onychophoran mouse millipede jelly

14. A Closer Look at the Arthropod Branches
a) How many body parts do insects have? _______ How many pairs of legs _______
b) How many body parts do chelicerates have? ____ How many pairs of legs _____Do chelicerates have antenna? ____
c) Do crustaceans have antenna? _________ Give two examples of crustaceans. _________________
d) Give two examples of myriapods: ___________________________________________
e) Name the 3 lobes of the trilobite body: ________________________________________

15. Meet the Cambrian Critters a) How long ago was the Cambrian period?____________________
b) What is the "Cambrian Explosion" _____________________________________________

16. Sanctacaris
a) Nickname __________________________
b) Is it part of the arthropod lineage? ________________________
c) Sanctacaris added as a branch to what part of the tree ___________________________

17. Opabinia
a) How did it acquire food? _________________________
b) Is it part of the arthropod lineage? ________________________
c) Opabinia lacks what namesake trait of arthropods? _____________________________

18. Pikaia
a) Is it part of the arthropod lineage? ________________________
b) Pikaia was probably what type of animal? __________________________

19. Hallucigenia
a) Is it part of the arthropod lineage? ________________________
b) Hallucigenia added as a branch to what part of the tree ___________________________

20. Naraoia
a) Is it part of the arthropod lineage? ________________________
b Naraoia added as a branch to what part of the tree ___________________________

21. Tools for Success - Crustaceans: Living Toolboxes . For each tool, name the arthropod that has a similar appendage.
Crowbar _____________________________________Paddle _____________________________________
Broom _____________________________________Leaf blower _____________________________________
Clamp _____________________________________Sledgehammer _____________________________________

22. Arthropod Adaptability - Describe two ways the legs of arthropods are adapted for different purposes.

26. Sidetrip: Extreme adaptations: Eating with your feet. Sketch the head of an insect and label the mouthparts

27. a) What is an evolutionary constraint?_____________________________________________
b) What are three constraints on arthropods? ____________________________________________

28. Why does molting limit an arthropod's size? ____________________________________________

29. How does strength limit the arthropod's size? ____________________________________________

30. a) How do insects get oxygen? ______________________________________________________
b) How does this limit their size? _____________________________________________________

Phylum Arthropoda

The name &ldquoarthropoda&rdquo means &ldquojointed legs&rdquo (in the Greek, &ldquoarthros&rdquo means &ldquojoint&rdquo and &ldquopodos&rdquo means &ldquoleg&rdquo) it aptly describes the enormous number of invertebrates included in this phylum. Arthropods dominate the animal kingdom with an estimated 85 percent of known species included in this phylum many arthropods are as yet undocumented. The principal characteristics of all the animals in this phylum are functional segmentation of the body and presence of jointed appendages. Arthropods also show the presence of an exoskeleton made principally of chitin, which is a waterproof, tough polysaccharide. Phylum Arthropoda is the largest phylum in the animal world insects form the single largest class within this phylum. Arthropods are eucoelomate, protostomic organisms.

Phylum Arthropoda includes animals that have been successful in colonizing terrestrial, aquatic, and aerial habitats. This phylum is further classified into five subphyla: Trilobitomorpha (trilobites, all extinct), Hexapoda (insects and relatives), Myriapoda (millipedes, centipedes, and relatives), Crustaceans (crabs, lobsters, crayfish, isopods, barnacles, and some zooplankton), and Chelicerata (horseshoe crabs, arachnids, scorpions, and daddy longlegs). Trilobites are an extinct group of arthropods found chiefly in the pre-Cambrian Era that are probably most closely related to the Chelicerata. These are identified based on fossil records.

Figure (PageIndex<1>): Trilobite fossil: Acadoparadoxides, possibly A. briareus, a large trilobite from about 500 million years ago from Morocco, North Africa (Middle Cambrian)

Several Precambrian animals of the Ediacaran fauna are thought to be either early arthropods or relatives of the first arthropods. There are 2 Early Cambrian animals which seem to be related to the first arthropods but belong to a group which is now extinct. Anomalocaris (which could reach over 11 cm) and Peytoia (which could be more than 2 m in length) were not arthropods, although they possessed a number of arthropod characterisitics such as a tough exoskeleton, molting, segmentation, their type of gill, and pivot joints in their appendages. These predators probably fed on trilobites (some trilobites have been found with wounds which are thought to have come from them).

A number of groups are considered as stem arthropods such as Fuxianhuia and its relatives, Canadaspis and its relatives, anomalocaridids, Opabinia, “gilled lobopodians” such as Kerygmachela, and lobopodians such as Megadictyon (Edgecombe, 2010).

In the early Cambrian, a number of basal arthropods are known such as Shankouia zhenghei, Fuxianhuia protensa, and Chengjiangocaris longiformis. It appears the arthropods first evolved segmented limbs and then began to develop a hard exoskeleton. In basal arthropods, the head was limited to those segments which bore the eyes and short, stout antennae. They are arthropods but not members of the crown group Euarthropoda, because they lack segments with post-oral limbs composing the head, the head is not covered by a shield fused to its segments (but rather a shield-like structure which is mot fused), and their limbs lack euarthropod modifications. They do possess euarthropod features which are absent in the arthropod relatives such as modern onychophorans and tardigrades and Cambrian tardipolypods. These features include compound eyes, a hard exoskeleton, segmented antennae, the possession of a shield-like protection of the head, segmented limbs, and other features (Waloszek, 2005). Fuxianhuia and Canadaspis are very close to the ancestral arthropod stem. They possess a body and legs organized into segments but lack the fusion of the head segments found in true arthropods (Maas, 2001).

The Early Cambrian arthropod Fuxianhuia has been classified both as an arthropod and as a wormlike organism more primitive than the true arthropods. It now appears that it is a basal arthropod with the primitive feature of separate head segments including a separate eye-bearing segment. The embryonic development of modern arthropods suggests that the arthropod head evolved from separate segments which fused together (Chen, 1995 Waloszchek, 2005)

A number of Cambrian groups seem to be related to arthropods, such as the pentastomids (tongue worms), onychophorans, tardigrades (water bears), and lobopodians (the latter is known only from the Cambrian) (Maas, 2001). The Xenusiana oR Lobopods, onychophore-like organisms, were first identified in the Burgess Shale with fossils of Aysheaia. Hallucigenia is also known from the Burgess Shale, from the Middle Cambrian of North America. Other known fossil genera include Luolishania, Cardioductyon, Hallucigenia, Microdictyon, Onychodictyon, Paucipodia (all from the Lower Cambrian of China), Xenusion (from the Lower Cambrian of Europe), and Hadranax (from the Lower Cambrian of Greenland). They share a segmented body plan, soft skin, and a common lobopod appendage with strong claws (one or two claws per appendage). The number of paired appendages varied from 9 in Paucipodia to 25 in Cardiodictyon (Bergstrom, 2001 Hou, 1995). One Middle Cambrian lobopod possessed three pairs of limbs (in addition to the stubs of a fourth pair) (Budd, 2001).

The brain of onychophorans is similar to that of invertebrates such as nematodes and priapulid worms (Budd, 2001). Fossil evidence and comparisons between arthropods and onychophorans suggest that the ancestral arthropod eyes were simple ocellus-like structures (Mayer, 2006).

Tardipolypodans lacked jaws and either preyed on microscopic organisms or were scavengers (Ivantsov, 2005).

Kerygmachela has a mix of both onchophoran and arthropod characteristics (Akam, 2000).

Lobopods are considered to be a sister group to arthropods. The lobopod Miraluoishania possessed features associated with arthropods such as paired eyes, antennae, and features of the arthropod head while other features such as its wormlike body and nonsegmented limbs are more primitive. The similarity of its antennae to those of true arthropods suggests that it is a transitional form in arthropod evolution. Megadictyon, with its segmentation and the possibility of a hard outer surface, also seems to be a transitional form between lobopods and arthropods (Liu, 2008).

A Cambrian lobopod from China, dating 500 million years old and measuring 6 cm, possessed 10 pairs of jointed legs (Dell’Amore, 2011).

Arthropods make up between 85% and 99% of modern species alive on earth. There are a number of groups of arthropods that were important in the Paleozoic. A bizarre modern group called onychophorans seem to link arthropods to segmented worms. Onychophorans were very successful in the Cambrian and include animals such as Aysheaia. A number of animals which had previously been classified as unknown groups now are classified as onychophorans (Levinton, 1992). Kerygmachela has a mix of both onchophoran and arthropod characteristics (Akam, 2000). The hemolymph of onychophorans contains the respiratory pigment hemocyanins, as do arthropods. Some genetic evidence supports the position of onychophorans as the sister group of arthropods whild other results classify them within the group Arthropoda (Kusche, 2002 Fortey, 1993)

Meristosoma, known from the Middle Cambrian, seems to be a basal form of arthropod, linking myriapods (arthropods which include millipedes and centipedes) to trilobites. It had a long segmented thorax but had anterior and posterior shields unlike the myriapods (Robison, 1995).

Trilobites are related to a few other groups of extinct arthropods which are classified together as the Trilobitomorpha. Some of these relatives, such as members of the Tetracephalosomita pictured below, possess some characteristics typical of trilobites (Edgecombe, 1998 Sharov, 1966).

Trilobites survived about 300 million years during which time they diversified into more 10,000 species (Lieberman, 2010). Trilobites were the most common and diverse group of the Cambrian. They were diverse afterwards as well945 genera and 56 families are known from the Ordovician and Siluran alone. Of the two main groups of trilobites in the Ordovician, one group became extinct in the mass extinctions at the end of that period while the second survived into the Siluran (with of its Ordovician members surviving the mass extinction) (Adrain, 1998). The predators which fed on trilobites included predatory arthropods such as Sidneyia and Utahcaris, cephalopods, and eurypterids (sea scorpions described below).

Trilobites disappeared during the Permian. The exoskeleton of trilobites was shed through a series of molts and many fossils are remains of molted skin rather than entire organisms. Trilobites fed on debris on the ocean floor as adults. The small size and the spines of the youngest juveniles suggest that they swam in plankton as they grew (Prothero, 1998).

Sea spiders (Pycnogonida) are generally thought to represent the most primitive and earliest chelicerates, dating back to the Late Cambrian. The first horseshoe crabs (Xiphosurans) and sea scorpions (eurypterids) are known from the Ordovician. The oldest known scorpions were aquatic, dating from the Silurian, with the earliest terrestrial species known from the Late Carboniferous. Harvestmen are closely related to scorpions with the earliest members known from the Devonian (Dunlop, 2010).

Although modern arachnids are primarily a terrestrial group (spiders), marine forms are known from the Cambrian.

Eurypterids or "Sea Scorpions"

About 200 species of sea scorpions have been identified. The earliest are known from the Late Ordovician and the last known specimens date from the Late Permian (a period of about 210 million years). While they were an aquatic group, their respiratory system probably would have accommodated short trips onto land. While some modified their appendages to become thin, walking stick-like structures, others formed flat swimming paddles. Prior to the Carboniferous, sea scorpions are limited primarily to northern continents although the large pterygotids were able to swim the ocean and spread throughout the world (Tetlie, 2007). A sea scorpion claw has been found of such size that it is estimated that it belonged to an animal 2.5 meters long (Williams, 2007).

Most eurypterids were larger than 1/10 a meter and at least one type reached 2.5 meters (8 feet) in length. They are known since the Ordovician and they were the main predators of the Siluran seas. In the Devonian Period they were replaced by large predatory fish and they became extinct in the Permian (Prothero, 1998).

Xiphosurans or "Horseshoe Crabs"

Xiphosurans were known from the Siluran to present. (Another group of arthropods which were once classified with xiphosurans are from the Cambrian.). They have never been overly diverse or abundant.

A few species of true crustaceans, such as Wujicaris muelleri, are known from the Lower Cambrian (Zhang, 2010). Primitive crustaceans with appendages from the Lower Cambrian support the conclusion that early steps in arthropod evolution occurred in the Precambrian (Siveter, 2001).Crustaceans are a diverse group of arthropods which include:

--decapods (crabs, lobsters, shrimp) which are known since Devonian and were extremely successful in Mesozoic

maxillopods: barnacles (known since the Ordovician) and ostracods (very small but the most commonly fossilized arthropods and thus very important in stratigraphy known since the Early Cambrian)

It is thought that freshwater crayfish evolved from marine lobsters at the beginning of the Mesozoic or just before. There are no known fossils of crayfish in southern continents until the Early Cretaceous (Martin, 2008).


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Arthropod, (phylum Arthropoda), any member of the phylum Arthropoda, the largest phylum in the animal kingdom, which includes such familiar forms as lobsters, crabs, spiders, mites, insects, centipedes, and millipedes. About 84 percent of all known species of animals are members of this phylum. Arthropods are represented in every habitat on Earth and show a great variety of adaptations. Several types live in aquatic environments, and others reside in terrestrial ones some groups are even adapted for flight.

The distinguishing feature of arthropods is the presence of a jointed skeletal covering composed of chitin (a complex sugar) bound to protein. This nonliving exoskeleton is secreted by the underlying epidermis (which corresponds to the skin of other animals). Arthropods lack locomotory cilia, even in the larval stages, probably because of the presence of the exoskeleton. The body is usually segmented, and the segments bear paired jointed appendages, from which the name arthropod (“jointed feet”) is derived. About one million arthropod species have been described, of which most are insects. This number, however, may be only a fraction of the total. Based on the number of undescribed species collected from the treetops of tropical forests, zoologists have estimated the total number of insect species alone to be as high as 5.5 million. The more than 48,000 described species of mites may also represent only a fraction of the existing number.

The phylum Arthropoda is commonly divided into four subphyla of extant forms: Chelicerata (arachnids), Crustacea (crustaceans), Hexapoda (insects and springtails), and Myriapoda (millipedes and centipedes). Some zoologists believe that arthropods possessing only single-branched appendages, particularly the insects, centipedes, and millipedes, evolved from a separate ancestor and therefore group them within a separate phylum—the Uniramia, or Atelocerata however, in this treatment these forms are dispersed among several subphyla. In addition, the phylum Arthropoda contains the extinct subphylum Trilobitomorpha. This group is made up of the trilobites, the dominant arthropods in the early Paleozoic seas (541.0 million to 251.9 million years ago). Trilobites became extinct during the Permian Period (298.9 million to 251.9 million years ago) at the end of the Paleozoic Era.

The myriapods (centipedes, symphylans, millipedes, and pauropods) live beneath stones and logs and in leaf mold insects are found in all types of terrestrial habitats and some have invaded fresh water. The sea has remained the domain of the crustaceans, however, and only at its very edges are insects (subphylum Hexapoda) found.

The subphylum Crustacea contains mostly marine arthropods, though many of its members, such as the crayfish, have invaded fresh water, and one group, the pill bugs (sow bugs), has become terrestrial, living beneath stones and logs and in leaf mold. In the sea, large crustaceans such as crabs and shrimps are common bottom-dwelling arthropods. Many minute species of crustaceans (particularly the copepods) are an important component of the zooplankton (floating or weakly swimming animals) and serve as food for other invertebrates, fishes, and even whales.

Most members of the subphylum Chelicerata belong to the class Arachnida, containing the spiders, scorpions, ticks, and mites. They are largely terrestrial arthropods, living beneath stones and logs, in leaf mold, and in vegetation, but there are some aquatic mites that live in fresh water and in the sea. There are also many parasitic mites. Two small classes of chelicerates, the Merostomata, containing the horseshoe crabs, and the Pycnogonida, containing the sea spiders, are entirely marine. The merostomes are an ancient group and probably gave rise to the arachnids. Indeed, the earliest known fossil scorpions were aquatic.

This article discusses the arthropods as a group. For specific information on the most significant subphyla and classes of arthropods, see crustacean arachnid insect. See also myriapod.




Figure 15. A centipede


Figure 16. Fly life history showing complete metamorphosis

Arthropods: Habitat, Classification and Phylogeny

The earliest record of the study of arthro­pods is available from the work of Aristotle (384-322 B.C.), who coined the term Malacostraca to include crabs and the re­lated forms.

The present trend of studying Arthropoda began with the work of Linnaeus (1707-1778), who created a group Insecta aptera to include Crustaceans, Myriapods and Spiders. The name Crustacea and Myriapoda were first introduced by Cuvier (1769-1832) and Latreille (1825).

Lamarck (1744-1829) in his classification, included spiders, mites, myriapods and silver fishes under Arachnida and grouped prawns, lob­sters, crabs and water fleas within Crustacea. It was Cuvier who first suggested to include these animals and annelids under one large group, Articulata. Von Siebold (1845) later separated the annelids and the rest were included under Arthropoda.

Definition of Arthropods:

Bilaterally symmetrical and metamerically segmented animals body covered with jointed chitinous exoskeleton, moulted periodically and with jointed segmental appendages coelom highly reduced and haemocoelomic body cavity.

Habit and Habitat of Arthropods:

The arthropods are seen from 30,000 feet below to 20,000 feet above the sea level. These bilaterally symmetrical, jointed-leg invertebrates may be marine, fresh-water, terrestrial, subterranean and aerial. Some arthropods like barnacles are sedentary.

In­numerable crustaceans which live as planktons move passively in the current of water. But well-developed structures are present in many arthropods for moving ef­fectively by swimming, crawling and flying.

Some arthropods live within burrows, some are efficient diggers and many others build well-designed nests. Certain arthropods like honey-bees, ants and termites are polymor­phic and lead a complicated social life. All the food habits—herbivorous, carnivorous and omnivorous are seen among arthropods and various food-getting devices are met within this group.

Large numbers of arthro­pods live as parasites, and structural changes occur in them to adjust with the peculiar mode of life. Many arthropods are well- known for their habit of migration. Some of them can produce sound and nearly all are equipped with efficient sense organs.

Some forms exhibit a phenomenon—suspended animation, to overcome unfavourable con­ditions. Sexual reproduction is often accom­panied by courtship dances. The members may either be oviparous or viviparous or ovoviviparous and some forms exhibit pa­rental care. Parthenogenesis is quite com­mon in arthropods.

Characteristic Features of of Arthropods:

1. Body is bilaterally symmetrical and is metamerically segmented, coelomates.

2. Anterior segments are specialized to form a distinct head and tagmatization (body region) is highly developed (e.g., head, thorax and abdomen).

3. Body is covered by jointed hard chitinous exoskeleton (usually com­posed of carbohydrate and protein) with sclerotised plates moulted periodically.

4. Presence of paired jointed segmental appendages.

5. Presence of musculature with distinct striped muscles.

6. Body cavity or true coelom is much reduced and acts as haemocoel (blood cavity).

7. Circulatory system is open type (e.g., blood vessels open within haemocoel) with a dorsally placed tubular heart containing paired lateral ostia.

8. Haemocyanin is the usual respiratory pigment.

9. Nephridia are represented by the paired saccular excretory organs (e.g., coxal glands, antennal or maxillary glands) of many arthropods. The Malpighian tubules act as excretory organs, found in many terrestrial arthropods.

10. Central nervous system includes a dorsally placed anterior brain and ganglionated double ventral nerve cord.

11. Presence of compound eyes in many groups (e.g., many crustaceans and most insects), in which each eye is composed of several visual units (e.g., ommatidia).

12. Dorsal coelomic gonads.

13. Sexes are usually separate (= gonochoristic) some hermaphrodite.

14. Parthenogenesis is seen in some mem­bers of Insecta, Branchiopoda and Copepoda.

15. Fertilization internal in terrestrial spe­cies but external in aquatic species.

16. Eggs are centrolecithal.

17. Cleavage commonly superficial a few spirals (e.g., Barnacles, copepods, cladocerans).

18. Development may be direct or indirect (e.g., via larval stages).

19. Absence of ciliated larvae.

Classification of Arthropods:

Classification in Outline:

In the studies of the middle part of 20th century, considerable rearrangement has been made in the scheme of arthropod classifica­tion.

The classification given below is based primarily on the studies of Vandel, A (In Traite de Zoologie, Tome VI, ed, P. Grassae pp. 79-158, 1949) and Snodgrass, R. E. (Ar­thropoda, in ‘McGraw-Hill Encyclopedia of Science and Technology’ Vol. I, McGraw- Hill, New York, 1960).

Phylum Arthropoda:

I. Subphylum Trilobitomorpha or Trilobita:

(i) Class Trilobita, e.g., Agnostus, Trinucleus.

II. Subphylum Chelicerata or Arachnomorpha:

1. Subclass Xiphosurida e.g., Limulus, Tachypleus.

2. Subclass Eurypterida e.g., Eurypterus.

Order Scorpionida e.g., Palamnaeus, Buthus.

Order Uropygi e.g., Mastigoproctus, Trithyreus.

Order Amblypygi e.g., Sarax, Myodalis, Tarantula.

Order Palpigradi e.g., Eukoenenia, Prokoenenia.

Order Araneida e.g., Araneus, Argiope, Lycosa, Latrodectus.

Order Ricinulei e.g., Cryptocellus, Ricinoides.

Order Pseudoscorpionida e.g., Garypus, Faella.

Order Solifugae e.g., Galeodes, Eremobates.

Order Opiliones e.g., Trogulus, Mitobates.

Order Acardia e.g., Trombicula, Argas, Boophilus, Dermacentor.

(iii) Class Pycnogonida e.g., Nymphon, Pycnogonum.

III. Subphylum Mandibulata:

Order Cephalocarida e.g., Hutchinsoniella, Lightiella.

Order Anostraca e.g., Artemia, Branchipus.

Order Notostraca e.g., Triops, Lepidurus.

Order Diplostraca e.g., Leptodora, Daphnia.

Order Mydocopa e.g., Philomedes, Cypridina.

Order Cladocopa e.g., Polycope

Order Platycopa e.g., Cytherella.

Order Podocopa e.g., Cypris, Darwinula.

4. Subclass Mystacocarida e.g., Derocheilocaris

Order Calanoida e.g., Calanus, Diaptomus

Order Harpacticoida e.g., Attheyella, Harpacticus.

Order Cyclopoida e.g., Cyclops, Ergasitus

Order Notodelphyoida e.g., Natodelphys, Doropygus.

Order Monstrilloida e.g., Monstrilla.

Order Caligoida e.g., Caligus, Eudactylina.

Order Lernaeopodoida e.g., Brachiella, Lernaea.

Order Branchiura e.g., Argulus, bolops.

Order Thoracica e.g., Balanus, Lepas.

Order Acrothoracica e.g., Cryptophialus, Trypetesa.

Order Apoda e.g., Proteolepas

Order Rhizocephala e.g., Sacculina, Peltogaster.

Order Ascothoracica e.g., Synagoga, Dendrogaster.

1. Super order Phyllocarida

Order Nebaliacea e.g., Nebaliopsis, Nebalia.

2. Super order Hoplocarida

Order Stomatopoda e.g., Squilla, Coronida.

Order Anaspidacea e.g., Anaspides, Paranaspides.

Order Bathynellacea e.g., Bathynella.

Order Mysidacea e.g., Mysis, Neomysis.

Order Cumacea e.g., Cumopsis, Diastylis.

Order Tanaidacea e.g., Tanais, Neotanais

Order Isopoda e.g., Oniscus, Ligia.

Order Amphipoda e.g., Gammarus, Caprella.

Order Euphausiacea e.g., Euphausia, Nematoscelis.

Order Decapoda e.g., Palaemon, Homarus, Palinurus, Scyllarus, Hippa, Eupagurus, Cancer.

Order Scutigeromorpha e.g., Scutigera.

Order Lithobiomorpha e.g., Lithobius.

Order Scolopendromorpha e.g., Scolopendra

Order Geophilomorpha e.g., Geophilus.

(iii) Class Symphyla e.g., Scutigerella, Scolopendrella.

(iv) Class Pauropoda e.g., Pauropus.

(v) Class Diplopoda e.g., Scutigerella, Scolopendrella.

(a) Subclass Pselaphognatha

Order Pselaphognathae e.g., Polyxenus.

Order Platydesmida e.g., Platydesmus.

Order Polyzoniida e.g., Polyzonium.

Order Polydesmida e.g., Polydesmus.

Order Chordeumida e.g., Chordeuma.

Order Spirobolida e.g., Spirobolus

Order Spirostreptida e.g., Thyropygus.

(vi) Class Insecta or Hexapoda

Order Protura e.g., Eosentomon, Acerentomen.

Order Collembola e.g., Isotoma, Neanura.

Order Diplura e.g., Campodea, Heterojapyx.

Order Thysanura e.g., Lepisma, Machilis.

Order Ephemeroptera e.g., Ephemera, Hexagenia.

Order Odonata e.g., Aeschna, Libellula, Ischnura.

Order Dictyoptera e.g., Periplaneta, Mantis.

Order Isoptera e.g., Termes, Odontotermes.

Order Zoraptera e.g., Zorotypus.

Order Plecoptera e.g., Perla, Isoperla.

Order Notoptera e.g., Crylloblatta.

Order Cheleutoplera e.g., Carausius, Phyllium.

Order Orthoptera e.g., Hieroglyphus, Tryxalis, Locusta Schistocerca, Gryllotalpa.

Order Embioptera e.g., Embia.

Order Dermaptera e.g., Forficula.

Order Coleoptera e.g., Photinus, Calandra, Adalia, Dineutus.

Order Megaloptera e.g., SialiSi Corydalis.

Order Raphidioptera e.g., Raphidia.

Order Planipennia e.g., Mantispa, Myrmeleon.

Order Mecoptera e.g., Panorpa.

Order Trichoptera e.g., Rhyacophilia.

Order Lepidoptera e.g., Parides, Papilio, Bombyx.

Order Diptera e.g., Anopheles, Musca.

Order Siphonaptera e.g., Pulex, Ctenocephalus.

Order Hymenoptera e.g., Apis, Vespa, Formica.

Order Strepsiptera e.g., Stylops.

Order Psocoptera e.g. Psocus.

Order Mallophaga e.g., Menopon.

Order Anoplura e.g., Pediculus.

Order Thysanoptera e.g., Heliothrips.

Order Homoptera e.g., Cicada, Aphis, Tachardia.

Order Heteroptera e.g., Cimex, Anasa, Triatoma.]

The classification, as given in the text book of Parker and Haswell (1972, 7th ed.), shows that the phylum Arthropoda is divided into seven subphyla.

Subphylum III. Pentastomida

Subphylum IV. Trilobitomorpha

Subphylum VII. Mandibulata

The classification of Arthropoda of Ruppert and Barnes (1994) as given partly in this textbook (4th ed.), shows that the phylum is divided into four subphyla.

They have upgraded the class Crustacea into subphylum rank and the subclasses under Crustacea have also been upgraded into class rank. Most recent zoologists are in favour to retain Crustacea as a class and Remipedia, Cephalocarida, Branchiopoda, Ostracoda, etc., keep them as subclasses rank.

These subphyla include the following classes:

Ruppert and Barnes (1994) have placed pentastomids as a sepa­rate class Pentastomida, related to Branchiurans and Copepods but Anderson (1998) has placed these parasitic worms in a sepa­rate phylum Pentastomida.

Classification with Characters:

1. Subphylum Trilobita (or Trilobitomorpha) [Gk. tri = three, lobos = lobe, morphe = shape = three-lobed form]

1. Extinct marine arthropods.

2. Body more or less oval and flattened from above downwards.

3. Body is divided into three regions:

(i) The anterior head or cephalon,

(ii) the middle region trunk or thorax and

4. Each region of the body is divided into 3 lobes by two longitudinal furrows, hence the animals derive their name Trilobites or three-lobed form.

5. Size varies from 10 mm to 60 cm.

6. Head and pygidium were covered by an un-jointed calcareous exoskeleton, called carapace.

7. Presence of a pair of compound eyes, found laterally on the anterior part of the body.

8. A pair of many-jointed antennae represents the pre-oral appendage.

9. Post-oral appendages are uniform, biramous and unspecialized. The in­nermost branch of each appendage was without long setae and was prob­ably adapted for walking and the outermost branch had long filaments used for swimming or filtering food materials. The two branches are some­times called endopodite and exopodite also.

10. Each leg has 8 segments.

11. The anal opening was on the last seg­ment of the pygidium.

The subphylum includes 4000 species which are grouped under 5 classes and the class Trilobita includes the largest number of species.

Agrestus, Ampyx, Mesonocis, Holmia, Trinucleus, etc.

2. Subphylum Chelicerata or Arachnomorpha [Gk. chele = claw]

1. Heterogenous group of arthropods, in all of which pre-oral antennules or first antennae are absent (nonantennate).

2. Body is divided into two parts— cephalothorax (or prosoma) and ab­domen (or opisthosoma) with no dis­tinct head.

3. Cephalothorax (or prosoma) possesses five postoral segments, each with a pair of appendages.

4. First pair of appendages on the first postoral segment is called chelicerae and are feeding appendages. The che­licerae become pre-oral in position. First pair of appendages is not an­tennae but chelicerae, used in feeding.

5. Chelicerates have no jaws (mandibles) hence may be called amandibulates.

6. Each chelicera is jointed and bears a terminal chela.

7. Abdomen (opisthosoma) consists of 12-13 segments and a telson (telson and many abdominal segments are absent in certain forms).

8. Second abdominal segment bears genital aperture which remains cov­ered by a modified abdominal ap­pendage, called operculum.

9. Compound eyes in most cases degene­rated.

10. Median simple eyes present.

11. Development usually direct.

12. Primarily marine arthropods, although most living forms are terrestrial.

It includes 3 classes:

Class 1. Merostomata [Gk. meros = the thigh stomatos, genitive of stoma = mouth]:

1. Marine forms with fairly developed compound eyes, present laterally.

2. Head and thorax are fused into a sin­gle unit—the prosoma or cephalotho­rax covered by a single sheet of ex­oskeleton, the carapace.

3. First pair of appendages on the prosoma is the Chelicerae followed by 5 pairs of appendages, the walking legs.

4. Prominent caudal spine, called telson, present at the end of the body, used as a lever in pushing and balancing dur­ing locomotion.

5. Respiratory organs are gills (book- gills), which are borne on the plate­-like appendages of the mesosoma.

6. Adults crawl on earth with the face downwards, but young can swim ac­tively.

The class is divided into two subclasses:

Subclass 1. Xiphosura [Gk. Xiphos = sword, oura = tail], Horse-shoe crab. Many fossil forms and 4 living species.

1. Cephalothorax (prosoma) is covered by a broad, smooth, and horse-shoe shaped carapace which is convex above and bears 2 pairs of eyes, one compound and lateral, and the other pair simple and median in position.

2. Caudal spine is elongated, slender and pointed.

3. Dorsal ridge is visible in the abdomen.

4. Abdomen (opisthosoma) bears 5 pairs of book-gills.

5. Excretion is performed by a four-lobed coxal gland.

6. Development with a larval stage, called trilobite larva.

7. These marine, bottom dwellers are commonly called horse-shoe crabs.

Limulus, Tachypleus, Carcinoscorpius.

Subclass 2. Eurypterida or Gigantostraca (Water scorpions)

2. Scorpion-like appearance.

3. Body is compressed dorsoventrally and protected by a chitinous exoskeleton.

4. Short prosoma is covered by dorsal carapace and consists of 6 segments fused together.

5. Trunk or abdomen is followed behind prosoma and consists of 12 free seg­ments.

6. At the end of the trunk or abdomen there is a post and tail-plate or spine which may be triangular (e.g., Eurypterus) or divided into two lobes (e.g., Pterygotus).

7. Mouth ventrally placed of the prosoma.

8. Five pairs of prosomal appendages except 1st pair are 6-8 joints each and are associated with locomotion.

Euryptarus (Ordovician- Permian), Pterygotus (Ordovician), Slimonia (Silurian).

Class 3. Arachnida [Gk. arachne = spider] Approx. 74,000 species.

1. Body divided into two regions— cephalothorax (Prosoma) and abdomen.

3. Compound eyes when present are degenerated.

4. Two pairs of jointed cephalic append- ages-chelicerae and pedipalpi present. The first pair of cephalic appendages, known as chelicerae, which are preoral and the 2nd pair, the pedipalps, are postoral and serve partly as jaws.

5. Four pairs of thoracic legs present.

6. Abdominal segments often reduced and abdominal appendages not asso­ciated with locomotion.

8. In terrestrial forms, the respiratory organs are book-lungs or tracheae or both in some species.

9. Excretory organs are malpighian tubules or coxal glands or both.

11. Eggs yolky and centrolecithal.

12. Development not accompanied by metamorphosis.

13. Carnivorous and mostly terrestrial.

The class Arachnida is divided into 10 orders:

1. These are terrestrial arachnids found under legs, stones or in rock crevices of tropical and subtropical regions.

2. Body is divided into prosoma (cephalothorax) and opisthosoma (ab­domen).

3. Prosoma unsegmented and covered by carapace bearing a pair of cheli­cerae, a pair of pedipalpi and four pairs of walking legs.

4. Abdomen divisible into two parts— broad pre-abdomen (mesosoma) con­sisting of 7 segments and narrow post- abdomen (metasoma) consisting of 5 segment with a caudal spine formed by the modification of telson.

5. Chelicerae are small 3-segmented but pedipalpi are large and 6-segmented.

6. Second segment of the pre-abdomen bears two comb-shaped pectines.

7. Four pairs of respiratory organs, called book lungs.

This order includes scorpions.

Palamnaeus, Mesobuthus, Buthus, Scorpio, etc.

1. These are commonly called whip scor­pions.

2. Size ranges from 2-65 mm in length.

3. Telson is present as a long whip-like flagellum at the posterior end.

4. Poison glands are absent but an acidic substance from the anus is sprayed.

5. Female provided with a brood sac to carry eggs.

6. Upper lip of the rostrum and the bases of the pedipalps form a peculiar ball and socket joint around the mouth to act as a filtering apparatus.

7. Short and stout pedipalp.

8. Second leg is antenna-like and elon­gated.

9. Abdomen bears 12 segments.

10. Book lungs placed on the second and third abdominal segments,

11. Nocturnal and carnivorous.

1. Commonly known as tailless whip scorpions.

2. Size varies from 4-45 mm in length.

3. Body flattened dorsoventrally.

5. Abdomen consists of 11 segments.

6. Chelicerae are 2-jointed and hook-like.

7. Pedipalps consist of seven segments and are stout and raptorial.

8. A prominent movable claw is present at the distal end of each pedipalp.

9. Book lungs open on the ventral side of second and third abdominal seg­ments.

10. Females carry brood-sacs during breeding season.

1. Commonly called microwhip scorpi­ons.

2. Size ranges from 0.5-3 mm in length.

3. In the cephalothorax last two segments are not united.

4. Telson with a many-jointed flagellum.

5. Chelicerae are 3-segmented and chelated.

7. Both simple and compound eyes are absent.

8. Second pair of legs act as antennae.

9. Respiration is cutaneous.

10. In addition, paired eversible sacs in the abdominal segments also act as respiratory organs.

1. Commonly called spiders.

2. Cephalothorax un-segmented.

3. Abdomen in general is also un-seg­mented, soft and round.

4. Four pairs of eyes are present.

5. Chelicerae are complex and poison glands open through them.

6. Pedipalpi simple and six-jointed.

7. Book lungs are associated with tra­chea for respiration.

8. Spinning glands usually present and appendages of fourth and fifth ab­dominal segment form spinnerets.

Argiope, Aranea (orb-web spi­der), Latrodectus, Lycosa (wolf-spider), etc.

1. Commonly called Ricinuleids.

2. Sizes usually less than 10 mm in length.

4. Cephalothorax is drawn anteriorly into a many-jointed movable projection, called cucullus.

6. Abdomen superficially looks like four segmented but truly it consists of nine segments.

7. Both the chelicerae and pedipalpi are chelated.

8. Tracheae are the respiratory organs.

Order 7. Pseudoscorpionida:

1. These are commonly called pseudoscorpions.

2. Size is never more than 8 mm in length.

3. Body oval and flattened dorsoventrally.

4. Abdomen broad and consists of eleven segments.

5. Abdomen is not separated into pre- and post-abdomen and does not bear the caudal sting.

7. Chelicerae are small and contain silk glands.

8. Pedipalpi is scorpion-like and contains poison glands.

9. Respiratory organs are tracheae.

10. Malpighian tubules are absent.

12. These are found under the bark of trees.

1. Commonly known as wind scorpions.

2. Size varies form 1-7 cm in length.

3. These are swift runners.

4. Body is divided in 3-regions—head, thorax and abdomen.

5. Abdomen oval and contains ten seg­ments.

6. Chelicerae large and prominent.

7. Pedipalpi elongated and resembles the legs.

8. Poison glands are absent.

9. Respiratory organs are well-developed tracheae.

10. All are nocturnal, ferocious and preda­tors.

1. Commonly called harvestmen.

2. Size varies from 1-22 mm in length.

3. Cephalothorax un-segmented.

4. Abdomen consists of 10 segments and is not distinctly separated from the cephalothorax.

6. Compound eyes are lacking.

8. Walking legs are usually very long and slender.

9. Respiration takes place through tra­cheae.

10. No poison or spinning glands.

11. Males possess copulatory organ and females are provided with ovipositor.

Caddo, Phalangium, Trogulus, Mitobates.

1. Commonly known as ticks and mites.

2. Number of forms is microscopic in sizes.

3. Body without any external division.

4. Chelicerae and pedipalpi are usually small and associated with the mouth- parts which are adapted for—biting, piercing or sucking.

5. Chelicerae are usually composed of 2 to 3 segments but may have up to 6 segments.

6. Chelicerae may be pincer-like, fang­like or lance-like.

7. Pedipalpi also may be variously modi­fied.

8. Legs are provided with claws.

9. Respiratory organs are tracheae or in many, respiration is cutaneous.

10. Most of them are parasites on man and other animals.

11. Some are notorious pests of agricul­tural products.

Tetranychus, Demodex, Hoplophorella, Annectacarus, Sarcoptes.

Class 3. Pycnogonida (= Pantopoda):

Approx. 1,000 species about 16 Indian species.

1. Partially sedentaric marine chelicerates, commonly called sea spi­ders.

2. Young’s are parasitic on different soft bodied invertebrates.

3. Externally segmented body.

4. Reproductive openings present on the leg segments and not abdominal.

5. Chelicerae short and pedipalpi seg­mented.

7. Third pair of appendages in the male carries the eggs and is called the ovigers.

8. Trunk of 3-6 segments with long walk­ing legs.

9. Opisthosoma much reduced with a terminal anus.

10. Protonymphon larva in the life cycle of most pycnogonids.

Nymphon, Pycnogonum, Colossendeis.

3. Subphylum Crustacea:

[L. crusta – a hard shell] Copepods, shrimps, prawns, barnacles, lobsters crabs. Approx. 45000 species about 3000 Indian species.

1. Body divisible into 3 regions—head, thorax and abdomen.

2. Two pairs of antennae are a distin­guishing feature among crustaceans.

3. Other cephalic appendages are a pair of mandibles and two pairs of max­illae.

4. Thoracic and abdominal appendages are usually 8 pairs and 6 pairs, respec­tively, variable in lower crustacea.

5. Appendages typically biramous ex­cept of antennules.

6. Carapace covers all or part of the body.

7. Head bears a pair of compound eyes on movable jointed stalk.

8. Respiration takes place either by gills or by the general surface of the body when the exoskeleton is thin or by some of the limbs.

9. Vascular system consists of a contrac­tile heart, arteries and haemocoelomic spaces.

10. Excretory organs are the modification of coelomoducts may be either antennal glands (green glands) or shell glands (maxillary glands found in the second pair of maxillae).

11. Brain formed by the fusion of first four embryonic ganglia and is con­nected with ventral nerve cord by oesophageal connectives.

13. Distinct sexual dimorphism present.

14. Eggs usually centrolecithal, i.e., yolk present in the central part of the egg, or may be telolecithal, i.e., yolk occu­pies one-half of the egg, or alecithal, i.e., without yolk.

15. Development includes a larval form, the nauplius, bearing a single median eye and 3 pairs of appendages.

16. Mainly aquatic, mostly marine, many freshwater and some have invaded into terrestrial condition.

The subphylum crustacea is divided into 11 classes:

Many zoologists, such as Ruppert and Barnes (1994) use the category subphyla:

(i) Crustacea for typical bi-ramous appendages (e.g., copepods, barnacles, shrimps, lobsters and crabs) and

(ii) Uniramia for uniramous ap­pendages (e.g., centipedes, millipedes and insects) instead of subphylum Mandibulata.

Pechenik (2000) and many zoologists agreed to use Mandibulata containing the classes— Crustacea, Myriapoda and Insecta.

He used Myriapoda (Gk. many feet) con­taining 4 orders, such as:

(i) Order Chilopoda (e.g., centipedes),

(ii) Order Diplopoda (e.g., millipedes),

(iii) Order Symphyla (e.g., symphylans) and

(iv) Order Pauropoda (e.g., pauropods).

The class Myriapoda is characterised by many segmented trunk, with each bearing uniramous legs, a pair of anten­nae, compound eyes absent and malpighian tubules for excretion.

Class 1. Remipedia:

This group was first recognised in 1983 with twelve known species.

1. Small and worm-like bodies, range up to 30 mm in length.

2. Head covered by a head-shield, followed by a trunk of 20-30 similar segments.

3. Each segment of the body bears a pair of lateral biramous appendages.

4. Telson with caudal rami.

They are the inhabitants of tropical marine caves.

Morphological data of this group suggest to be the clos­est of all the animals to the ancestral crustacean body but molecular data remain ambiguous.

Class 2. Cephalocarida [Cephalocarids Approx. 9 species]:

The members of this group are consi­dered to be most primitive among living crustaceans and the first member was dis­covered in Long Island Sound in 1955. The all species are marine and have collected in the soft sediments of the bottom up to the depths of over 1,500 m.

1. Small-sized animals exceeding 3.7 mm in length.

2. Horse-shoe shaped head followed by an elongated trunk.

3. First 8 trunk segments bear biramous appendages which are identical in ap­pearance.

4. The appendages are tripartite.

5. Exopodites of these appendages are four-jointed and leaf-like and bear lateral pseudoepipodite.

6. Endopodites are segmented, cylindri­cal and ambulatory in function.

7. Movements of the limbs produce wa­ter current for locomotion and also for collecting food.

8. Eyes are buried in the head.

Class 3. Branchiopoda (Gk. branchiona fin) Approx. 800 species:

1. Mostly freshwater species, a few are marine.

2. Trunk appendages are uniform and leaf-like.

3. Presence of one pair un-jointed or jointed caudal styles.

4. Carapace either absent or shield-like or bivalve.

5. First antennae and maxillae are small and in some cases absent.

6. Mandibular palp either rudimentary or absent.

It has 3 living orders:

1. It includes forms like fairy shrimps and brine shrimps.

3. They possess stalked eyes.

4. Antennae redudced and triangular in females.

5. In males, the antennae are stout copulatory structures.

6. Trunk is elongated and first 11 seg­ments bear alike legs.

7. Caudal styles un-jointed.

1. Commonly called Tadpole shrimp.

2. Carapace is large and shield-shaped.

3. Possesses 35-71 pairs of legs.

4. Eyes are sessile and placed close together.

6. Caudal styles filamentous and many- jointed.

7. Parthenogenesis is frequent.

Order (iii) Diplostraca:

1. It includes forms like clam-shrimps (Estheria) and water fleas (Daphnia, Leptodora).

2. Carapace laterally compressed, bivalved and enclosing trunk.

3. Biramous and large antennae are used during swimming.

4. Caudal styles clawed and un-jointed.

Class 4. Ostracoda (Gk. ostrakodes – testaceous resembling a shell) Approx. 7000 species:

1. Small crustaceans and commonly called seed-shrimps.

2. Mostly marine or freshwater, a few in terrestrial habitats.

3. Body enclosed in a hinged bivalved carapace.

4. Trunk appendages never more than 2 pairs.

6. Both pairs of antennae modified for swimming.

7. Respiration usually cutaneous.

8. Eyes may or may not be present.

9. Males are rare and the second anten­nae of the males serve as clasping organs.

The class Ostracoda includes 4 orders:

1. Carapace bears an aperture through which the antennae protrude.

2. Antenna is operated by powerful muscles.

4. Heart with paired ostia.

1. The aperture for the protrusion of an­tennae is absent.

2. Each second antenna bears two branches or rami.

1. Antennal aperture absent.

2. Rami of second antennae broad.

1. Antennal aperture on the carapace absent.

2. Endopodite of the second antenna well developed than exopodite and carries a claw.

3. Mandibular palp composed of four articles.

4. Four pairs of limbs are posteriorly placed.

Class 5. Copepoda (Gk. kope = handle) Approx. 8,500 known species:

1. Mostly small crustaceans.

2. Body with well-marked segments.

3. Trunk composed of a thorax bearing 5 pairs of biramous appendages used for swimming.

4. Abdomen without appendages.

5. Presence of a pair of caudal styles.

6. Head-shield present but no carapace.

7. Single median nauplius eye present but paired compound eyes absent.

8. Well-developed antennae may or may not be used for swimming.

9. Seventh segment of the body bears the reproductive apertures.

It includes 7 orders:

1. Free-living and size is fairly large.

2. Posterior part of the trunk is distinctly separated from the anterior part and between genital and pregenital seg­ments.

3. First antennae of female have 23-25 segments.

4. The second antenna is biramous.

5. The female possesses a single median egg-sac.

Canthocalanus, Eucalanus, Calanus, Paracalanus, Acrocalanus, Centropages, Phyllodioptomus, Heliodioptomus, Allodioptomus, Temora, Candacia, Calanopia, Acardia, Tortanus, Diaptomus.

Order (ii) Harpacticoida:

2. Trunk not constricted in the middle.

3. First antenna in the female is short and consists of 5-9 segments.

4. Second antenna is biramous.

2. Posterior part of the trunk is separated from the anterior part and it includes pregenital segments.

3. First antenna of female has more than 17 articles.

4. Second antenna is uniramous.

5. Female carries a pair of egg-sacs.

6. Some members are parasitic.

Cyclops, Ergasilus, Eucyclops.

Order (iv) Notodelphyoida:

1. Only in males the posterior half of the trunk bears pregenital segments.

2. Both sexes live as commensal within a tunicate, Ascidia.

Order (v) Monstrilloida:

1. Free-swimming and completely marine.

2. Adults without mouth parts, antennae and alimentary canal.

3. Larva starts as a free-swimming nauplius and parasitises a mollusc or annelid.

4. It draws nourishment by its antennae from the host.

5. Adult, when full grown, becomes again free-swimming and perform only re­production.

1. Lives as ectoparasite on fishes.

2. Posterior part of trunk carries two pregenital segments.

3. The antennae are modified to act as adhesive organs.

Order (vii) Lernaeopodoida:

1. Adults remain as ectoparasites up to the attainment of sexual maturity.

2. The sexually matured forms become free-swimming.

3. After copulation, female again be­comes parasitic on fishes.

4. It changes into a worm-like form and carries the eggs in a brood chamber.

5. Larvae start as free-swimming nauplius but soon infect a new host.

Class 6. Mystacocarida —Approx. 8 known species:

This subclass was created after the dis­covery of several crustaceans in the year in 1943.

1. Marine and interstitial.

2. Length of the body always within 1 mm.

3. Cylindrical bodies with distinct ce­phalic appendages.

4. Trunk consists of 5 segments each with a pair of appendages.

5. Nauplius eye persists and the com­pound eyes absent.

6. Two caudal styles work as pincers.

Class 7. Branchiura —Approx. 130 known species:

2. Dorsoventrally flattened body with suctorial mouth.

3. Broad shield-like carapace covers the cephalothorax.

4. Small, un-segmented and bilobed ab­domen.

5. Sessile compound eyes present.

6. Flagella present in the appendages of some body segments.

7. 5 pairs thoracic appendages.

8. Fifth body segment bears the genital apertures.

9. Males have two testes but females possess a single ovary.

10. Commonly called fish lice.

It includes a single order, having the same name and includes a single family.

Class 8. Pentastomida—Approx. 100 known species:

1. All the members are parasitic and live mainly in the lungs and nasal pas­sages of reptiles, but some species parasitize amphibians, birds and mam­mals including dogs and man.

2. Worm-like body ranges 2 to 13 cm long, of which the females are 10 cm in length.

3. Larva possesses 2-3 pairs of un-jointed Legs.

4. Adults are legless but possess only 4 pairs of anterior chitinous hooks, used for clinging to the host tissues.

5. Body covered by a non-chitinous cuti­cle and exhibits annular markings over the abdomen in the adult.

6. Exoskeleton moulted periodically.

7. Muscles are striated and metamerically arranged.

8. Most of the systems, such as digestive, excretory and reproductive are modi­fied to adapt the endoparasitic life.

9. Completion of the life history requires intermediate host.

10. They are gonochoristic, i.e., the sexes are separate.

11. Fertilization internal.

13. Pentastomids are popularly known as Tongue worms or sometimes referred to as “five mouths”.

The taxonomic status of Pentastomids has long been uncertain. Previously this group was treated as a separate phylum. But recently the sperm ultrastructure and analyses of DNA sequences coding for 18s ribosomal RNA indicate the similari­ties with crustaceans and suggest that Pentastomids are closely related to ma­rine crustaceans, specially with branchiurans and copepods.

Raillietiela, Cephalobaena, Linguatula, Armillifer.

Class 9. Tantulocarida—Approx. 5 known species:

1. Ectoparasite on other deep water crus­taceans.

3. Appendages absent in trunk segments.

Class 10. Cirripedia (L. cirrus = curled, pedis = foot). Approx. 1000 known species:

4. Six pairs biramous filamentous ap­pendages present.

5. Abdomen almost absent, with only a pair of caudal style.

6. Body enclosed within a bivalve cara­pace with calcareous plates on it.

7. Adults without eyes and antennae.

9. Young passes through nauplius and cypris stage.

10. Commonly called barnacles.

This subclass includes 5 orders:

1. Adults are permanently fixed by its preoral region on substrates at inter- tidal level.

2. Presence of six pairs of appendages in the trunk called cirri.

3. Abdomen without segments.

4. Some forms have a stalk, others are sessile.

Examples are Lepas (Goose barna­cles), Balanus (Acorn barnacles).

Order (ii) Acrothoracica:

1. Sessile forms and are fixed on the shell of molluscs.

2. Mantle devoid of calcareous plates.

3. Presence of only four pairs of ap­pendages in the trunk.

4. Male lives as parasite within female.

1. Completely parasitic forms.

2. Small distinctly segmented body.

3. Trunk appendages absent.

Order (iv) Rhizocephala:

2. Mantle remains, but the calcareous shells are absent.

3. Body absolutely degenerated in adults due to the loss of alimentary canal and appendages.

4. No trace of segmentation.

Order (v) Ascothoracica:

1. These are endoparasites on Anthozoa and Echinodermata.

2. Mantle bilobed or sac-like but plates are absent.

3. Oral appendages are modified for piercing and sucking.

4. Presence of 6 pairs of appendages in the trunk.

Symagoga, Laura, Dendrogaster.

Recent trend of the crustacean classi­fication shows that the subclasses Mystacocarida, Copepoda, Branchiura, Tantulocarida and Cirripedia are included under the class Maxillopoda for the characteristic fea­tures—6 thoracic and 5 abdominal segments and the first pair of trunk appendages are maxillipeds.

Class 11. Malacostraca (Gk. malakos = soft + ostracon = a shell). Over 20,000 species:

1. Body consists of 20-21 segments.

2. Thoracic and abdominal appendages distinct from one another.

3. Carapace covers the head and at least some thoracic segments.

5. Presence of compound eyes on stalk.

6. Antennule with two-many-jointed flagella.

7. Male and female gonopores on the bases of 8th and 6th thoracic append­ages.

The class includes five super orders:

Super order (i). Phyllocarida (Gk. phyllon = a leaf, L. caridis, genitive of caris = a shrimp):

1. A movable rostrum present at the anterior end of cephalothorax.

2. Carapace large, bivalved and encloses both the cephalothoracic and abdomi­nal segments.

3. Thoracic legs are all alike and folia- ceous.

5. Well-developed appendages in the first four abdominal segments.

6. Telson with a movable caudal furca.

It includes only one order Nebaliacea.

Presence of seven ab­dominal segments. Telson with a pair of caudal styles. A prominent carapace covers almost the entire length of the body. Thoracic appendages are leaf-like and serve as respi­ratory surface. Abdominal appendages are biramous. The common example is Nebalia.

Super order (ii). Hoplocarida (Gk. hoplon = a weapon):

2. Carapace flat, shield-shaped and en­closes the second thoracic segment.

3. Two movable segments lie anterior to the carapace.

4. Anterior one bears stalked eye and the posterior one carries the antennules.

5. Each antennule has three rami.

6. Antenna smaller than antennule.

7. Second pair of thoracic appendages is raptorial and bear a blade-like edge at its distal end.

8. Biramous abdominal appendages.

It includes only a single order Stomatopoda.

Last four thoracic segments are free from the carapace. Two movable segments are present in front of the head. Gills are carried by the abdominal appendages. Gastric glands extend up to the telson. Heart extends up to abdomen and has thirteen pairs of ostia. The well-known ex­amples are Scjuilla, Pseudoscjuilla and Coronida.

Super order (iii). Syncarida (Gk. syn = together):

2. Elongated and tube-like heart.

3. Only the first pair of thoracic append­ages modified as maxillipeds, rest are alike.

4. Biramous thoracic appendages.

5. Gills present on the thoracic append­ages excepting the last one.

6. Last pair of abdominal appendages, called uropods, is fan shaped.

It includes two orders:

1. First thoradc segment is fused with head.

2. They are inhabitants in the freshwater of ponds, streams of Australia, New Zealand and South America.

Order (ii) Bathynellacea:

1. They are the smaller than the mem­bers of Anaspidacea.

2. First thoracic segment is not fused with the head.

3. They are living in freshwater sediments and are world-wide in distribution.

Super order (iv). Peracarida (Gk. pira = a pouch):

1. Carapace may or may not be present. When present, carapace never covers last four thoracic segments.

2. Coxopodites of thoracic appendages bear a brood-pouch in females.

3. Presence of a tube-like, elongated heart.

This super order includes 5 orders:

1. First antennae are biramous.

2. Second antenna is with scale-like sq­uama.

3. Possesses filter feeding mechanism.

4. First pair of thoracic appendages is modified as maxillipeds.

5. Carapace is the chief respiratory sur­face.

6. Broad tail fin is formed by flat uropods and telson.

7. Heart is elongated but extends up to thorax and has two pairs of ostia. The common Examples are Mysis, Hemimysis.

1. Cephalothorax is posteriorly narrow.

2. Carapace is drawn out anteriorly to form rostrum and ventrally to form gill-chamber.

3. Sessile eyes are usually fused to form a single eye.

4. Antennae are usually un-segmented.

5. Second antennae are without exopodites and well developed in males than in females.

6. Abdomen is slender and segmented.

7. Abdominal appendages are absent in the female.

8. Uropods are rod-shaped and thus fan- shaped tail fin is absent.

The well-known examples are Cumopsis, Diastylis and Pseudocuma.

1. Carapace covers first two thoracic seg­ments.

2. Eyes, when present, are mounted on immovable stalks.

3. Second thoracic appendages are large and chelated.

4. A small squama may be present with the second antenna.

The examples are Tanais, Apseudes and Neotanais.

1. It includes aquatic, terrestrial and parasitic forms.

2. Carapace is absent, only first thoracic segment is fused into head.

3. Eyes are either without stalk or they are carried on small immovable proc­esses.

4. Body is dorsoventrally flattened.

5. Antennule is small and rudimentary.

6. First pair of thoracic appendages is modified as maxillipeds, while the others are alike.

The well-known examples are Anthura, wood borer (Limnoria), the ectopara­site of the fish, the Cymothoidae parasites on the gills of shrimps and crabs (Bopyrus), wood lice (Ligia, Liriopsis, Oniscus and Tylos, Adinda, Agnara, etc.).

Oniscus is an example of terrestrial crustacea). The wood bor­ing isopods, Limnoria lignorum, make deep funnels in wooden harbour for sheltering their teeming animals. They can also damage the submarine cables.

1. Body is flattened laterally.

3. Antennules are well developed and biramous.

4. Second and third pairs of thoracic appendages are prehensile structures, called gnathopods.

5. Some thoracic appendages bear gills at their bases.

6. Abdominal appendages are of two distinct morphological forms.

The examples are Gammarus, Caprella.

Super order (v). Eucarida (Gk. eu = true, L. caridis, genitive of caris = a shrimp):

1. Carapace covers head and all the tho­racic segments.

2. Mandible without sharp blade.

4. Small bag-like heart, placed on the dorsal side of the thorax.

Eucarida has two orders:

1. Thoracic appendages do not form maxillipeds and are all alike.

2. Single gill is present at the base of each thoracic appendage.

4. In the males first two pairs are modi­fied for copulation.

5. Elongated telson bears a movable large spine. Uropods are elongated.

The examples are krill (Euphausia, Nematoscelis, etc.), and all the mem­bers are marine, pelagic, shrimp-like animals, and have a world-wide dis­tribution. In the Southern Ocean they constitute a major food source of Baleen whales.

Order (ii) Decapoda (Gk. deka = ten, podos = foot):

1. Three maxillipeds are formed by the modification of first three thoracic appendages.

2. Three sets of gills are present which differ in their arrangements.

This order is divided into 2 suborders:

Suborder Dendrobranchiata:

Gills are dendrobranchiate type. The body is almost laterally compressed. The examples are penaeid shrimps (e.g., Penaeus, Funchalia, Parapenaeus, Metapenaeopsis, Metapenaeus), sergestid shrimp (Sergestes, Lucifer, Acetes).

The penaeid shrimps (fam. Penaeidae) are characterised by the well-developed and toothed rosturm, carapace without postorbital spine, 3rd and 4th pairs of pleopods biramous and telson sharply pointed with or without spines. They are found in sandy, mud estuaries, back water and near shore areas. The penaeids are the most valuable commercial shrimps exploited in many parts of the world.

A list of some Indian penaeid shrimps is given below (Table 18.13).

The other families of commercial shrimps are Solenoceridae, Aristeidae, and Sicyoniidae. The sergestid shrimps (Fam. Sergestidae) are characterised by small size, short rostrum and last 2 pairs of legs are shorter or absent. The examples are Sergestes, Lucifer, etc.

Gills are phyllobranchiate or trichobranchiate type. It includes 6 infraorders—Stenopodidea, Caridea, Astacidea, Palinura, Anomura and Brachura.

The three Caridean families:

Pandalidae, Cragnonidae and Palaemonidae are more commercially important and are found in seas and brackish waters. The Caridea includes snapping shrimps (Fam. Alpheidae, e.g., Alpheus), sand shrimps (Fam. Crangonidae, e.g., Crangon), Cock shrimps (Fam. Hippolytidae, e.g., Hippolysmata), Pandalid shrimps (Fam. Pandalidae, e.g., Helerocarpus), Palaemonid shrimps or Prawn (e.g., Palaemon, Macrobrachium, Leander, Brachycarpus, etc.).

In India the members of Palaemonidae are highly commercially important and char­acterised by the carapace cylindrical with laterally compressed rostrum, 1st pair of legs shorter and more slender than the 2nd pair, telson elongated with 2 pairs of dorsal and 2 or 3 pairs of posterior spines, and exopods absent on the legs. They are found in the freshwater ponds, canals, brackish water and seas.

The freshwater prawn, Macrobrachium rosenbergii, sometimes indi­cated as Palaemon carcinus auc non (Linnaeus, 1758), is found mainly in freshwater ponds, canals, rivers and also in brackish water. They are called the Golda chingri in Bengal.

Other important commercial species in India are Macrobrachium malcomsonii and M. rude. The monsoon river prawn, M. malcomsonii is found in rivers and estuarine areas of both coasts of India and Bangladesh. The species is extensively fished in the Chilika Lake, Orissa (India).

The Cray fish, Lobsters and Lobsterettes belong to the infraorder Astacidea. It is characterised by the well-developed abdo­men, first 3 pairs of walking legs with pin­cers and especially its first pair usually en­larged. The examples of lobsters are Acanthacaris, Metanephrops and Homarus, etc. and freshwater crayfish are Astacus and Cambarus.

The prickly deep sea lobster, Acanthacaris tenuimana (Fam. Nephropidae) is a deep sea lobster, found in west coast and Andaman lobster Metanephrops andamanicus (Fam. Nephropidae) live between 200 m and 750 m and has some commercial importance. Homarus is an American lobster.

The Infraorder Palinura is characterised by carapace more or less cylindrical, abdo­men somewhat flattened and legs may be chelate or sub-chelate. The spiny lobsters (Fam. Palinuridae) are characterised by with­out median rostrum but carapace with spines. The first pair of limbs is greatly enlarged and first four walking legs are without chelate.

The examples are Palinurus, Panulirus, Puerulus, etc. which is found in rocky and gravel bottoms. In India Panulirus ornatus and Panulirus polyphagus are the more or less common spiny lobsters, found in some pock­ets of west coast and east coast. They are commercially valuable species.

The Slipper lobsters are included under the family Scyllaridae which are character­ized by the carapace usually granular, some­times with spines and without rostrum. The flattened body and all walking limbs are without chelate. Examples are Thenus, Ibacus, Scyllarus, etc.

The flathead lobsters, Thenus orientalis is found in east and west coast of India and are the inhabitants of sandy or muddy sea bottoms. The locust lobster, Scyllarus orientalis is found in the south west coast of the Arabian Sea and inhabitants of sandy or muddy bottoms.

The Infraorder Anomura is characterized by depressed carapace and abdomen more or less reduced. Examples are hermit crabs, Diogenes, Pagurus, Eupagurus, Petrochirus, Dardanus, robber crab or coconut crab (Birgus), mole crabs, Emerita, Hippa, etc.

The coconut crab, Birgus latro is distrib­uted in the Indo-Pacific Islands and can climb to the top of the tallest coconut trees to pluck the coconuts. They can un-husk the outer fibrous layer of the coconut and eat the inner endocarp part (white part) by crushing the hard shell with the help of chela.

Lithodes, Paralithodes—commonly called king crabs because of their size among the largest crustaceans, are found in the North Pacific. The mole crabs are found in the wave- swept sandy beaches of the tropical seas.

The hermit crabs generally take refuge in the empty molluscan gastropode shells.

The Infraorder Brachyura is character­ized by the broad carapace, abdomen greatly reduced and permanently flexed beneath the body. The eyes are retracted into the cavities.

Examples are spider crabs (Maja, Arcania, Macrocheira), cancer crabs (Cancer sp.), swimming crabs (Portunus sp. Carcinus sp., Scylla sp.), pea crabs (Pinnotheres sp.), mud crabs (Pilumnus sp., Neptunus sp., Charybdes sp., Varuna sp.), freshwater crabs (Paratelphusa sp., Potamon sp.), ghost crabs (Ocypode sp.), fiddler crabs (Uca), grapsid crabs (Sesarma sp., Crapsus sp.).

The giant spider crab (Macrocheira kaemferi) is found in the coasts of Japan and the size may reach as much as 4 m between the tips of its legs.

Scylla serrata is found in the brackish water region in India. This crab is mainly found in the deltaic regions of Bengal and is called ‘Nona Kakra’ and consumed by the local people. They are in great demand in the markets of kolkata and other towns, and mainly supplied from the Canning areas (near Kolkata). Other brackish food crabs are Portunus sanguinolentus, Portunus pelagicus and Varuna literata.

The crab Scylla is identi­fied by smooth carapace, hand smooth and inflated, body regions are not well differen­tiated and anterolateral borders are marked by 9 large teeth. The food crab, Portunus is characterized by the divided carapace, hand prismatic and costate, and antero-lateral borders with 9 large teeth.

The freshwater crab, Paratelphusa is identified by the absence of swimming legs or pleopods and broad abdomen. Paratelphusa mansomana, Paratelphusa guerini and Paratelphusa spinigera, etc. are found in the freshwater habitat in different parts of India. Paratelphusa is consumed as a food and as a cure for common cold.

The bushy crab, Pilumnus is character­ized by moderately convex carapace and lateral border with sharp teeth. Pilumnus hirsutus, P. virensis, P. cursor, P. caerulescens and P. longicornis are found in Andamen and Nicobar Islands, and P. vespertilio is found in Ross Island (Andaman) and Krusadai Island (Tamil Nadu). Graspid crabs, Separma sp. live in burrows in the depth of 2 m.

The ghost crabs Ocypode ceratophthalma live in the burrows above the highly tide mark on the upper sandy beaches, frequently found in the shore of Digha, Sankarpur (W. Bengal), Talsari (Orissa) and other places in India. They are called ‘Red crab’ (Lai Kakra) in W. Bengal.

They are extremely agile and sometimes they are present by the hundred. The brilliantly coloured ‘Sally light foot crab’ Grapsus grapsus is seen over the black rocks along the shore in the Galapogos. They are also found in the Pacific coast of the American mainland.

4. Subphylum Uniramia:

2. Mandibles un-jointed and without palp.

3. Presence of a single pair of antennae.

4. Gas exchange takes place with the help of tracheal system.

5. Excretory organs are Malpighian tubules.

The subphylum is divided into 5 classes:

Class 1. Chilopoda [Gk. Cheilos = a lip]:

Approx. 3000 known species about 100 Indian species.

1. Body usually dorsoventrally flattened.

2. First pair of trunk appendages modi­fied as maxillipeds and work as poi­son claws.

3. Most of the trunk segments bear a single pair of uniramous walking legs.

4. Number of legs varies from 15 to more than 100 pairs but no form possesses even number of pairs.

5. Head bears a pair of antennae, a pair of mandibles and two pairs of maxillae.

6. Segment in front of telson is called genital segment.

7. Usually genital segment bears a pair of gonopods, help in reproduction.

8. Respiration takes place by trachea.

9. Excretion by a pair of Malpighian tubules.

11. Nocturnal and stay in humid areas.

12. Generally called centipedes or hundred-leggers.

13. Terrestrial, surface dwellers or some burrowers.

Geographical distribution:

They are distributed throughout the world both in tropical and temperate regions.

Four well-known orders:

(iii) Scolopendromorpha and

(iv) Geophilomorpha are placed within this class.

Order (i). Scutigeromorpha:

1. Legs are 15 pairs and very long.

2. Compound eyes are present.

3. Antennae are very long and originate from the posterior region of the ante­rior border of head.

4. Dorsal side of the head is arch-shaped.

5. Almost all the trunk segments bear a median spiracle, on their dorsal side. The common example is Scutigera.

Order (ii). Lithobiomorpha:

1. Legs are 15 pairs but very short.

2. Antennae arise from the anterior bor­der of the head.

3. Head and trunk both are flattened on the dorsal side.

4. Spiracles are laterally placed. The example is Lithobius.

Order (iii). Scolopendromorpha:

Strongly built body carries 21-23 pairs of legs only anterior part of the trunk bears lateral spiracles.

On the dorsal side of the trunk long plates alternate with shorter ones. The example is Scolcrpendra.

Order (iv). Geophilomorpha:

1. Number of legs varies from 35-181.

2. Body is narrow, worm-like and the legs are small.

4. Spiracles are placed laterally. The example is Geophilus.

Class 2. Symphyla (Symphylans):

Approx. 160 known species. About 4 In­dian species.

1. Mouth parts are directed forward.

2. Trunk composed of 12 legs bearing segments, covered by 15-24 terga.

3. Second maxillae are united to form the labium, similar to the insect.

5. The penultimate segment bears a pair of sensory bristles and a pair of spin­nerets.

6. Genital openings are located on the fourth trunk segment.

8. Spiracles present only in the head and trachea extends posteriorly only up to first three anterior trunk segments.

9. It includes herbivorous and omnivo­rous forms.

10. They are terrestrial, live in soil or leaf litter, found throughout the world.

Class 3. Pauropoda (Pauropods):

Approx. 500 known species probably no known Indian species.

1. Length rarely exceeds 1 mm.

3. The floor of the preoral chamber is formed by the fused pair of maxillae, called the gnathochilarium.

5. Trunk contains 12 segments.

6. Heart and tracheae (except in some primitive species) absent.

8. Legs are present in segments second to tenth.

9. Gonopores on 3rd trunk segment.

10. Saprophytic, mainly found in forest litter.

They are distributed both in tropical and temperate regions.

Class 4. Diplopoda (Millipedes) [Gk. diplos = double]:

Approx. 10,000 described species.

1. Elongated and segmented forms.

2. Trunk with a large number of leg- bearing segments.

3. First trunk segment (collum) is leg­less and next three segments with a single pair of legs in each segment and the rest doubled segments (diplosegments) bear 2 pairs of legs in each segment.

5. Maxillae are united to form gnathochilarium.

6. Tracheae are mostly un-branched tubes.

7. Gonads unpaired but reproductive ducts are paired.

8. Gonopores on the second pair of legs.

9. Usually vegetarian and found usually beneath leaves, logs, bark and stones.

10. They are commonly called Millipedes or thousandleggers

11. They are terrestrial and are mainly distributed in the tropics.

It includes two subclasses— Pselaphognatha (= Pencillata) and Chilognatha.

Subclass 1. Pselaphognatha (Pencillata):

1. The size of the body is very small.

2. Body is soft due to the absence of hard exoskeleton.

3. Gonopods are absent in males.

4. Head contains trichobothria.

5. Integument is often armed with lat­eral setae, hairs or bristles.

It includes a single order Pselaphognathae and the example is Polyxenus.

Subclass 2. Chilognatha:

1. The integument is provided with hard exoskeleton.

2. Head has no trichobothria.

3. Setae are not clustered.

5. Presence of gnathochilarium.

The subclass includes seven orders:

1. All parts of the gnathochilarium are present.

2. A prominent groove is present in the middle line of the dorsal surface. The example is Platydesmus.

1. Only a triangular plate is present in the gnathochilarium, other parts are obscure.

2. No median groove is present on the dorsal side.

The example is Polyzonium.

1. The segments vary from 18-22.

2. Lines of fusion between the exoskeletal plates are indistinct.

3. The dorsal plate, tergum, projects lat­erally as parnota.

5. Only the first pair of legs in the sev­enth segment are modified as gonopods.

The example is Polydesmus.

1. Number of segments is always more than thirty.

2. Ventral plates are separated by su­tures.

4. Both the pairs of legs in the seventh segment act as gonopods.

5. Last segment bears 1-3 pairs of spin­nerets.

1. Spinnerets are not present on the last abdominal segment.

2. Both pairs of legs of the seventh seg­ment are modified as gonopods in some cases one pair may be absent. The example is Julus.

1. Gnathochilarium is free from men- tum.

2. Only one pair of legs is present in the fifth segment.

The example is Spirobolus.

Order 7. Spirostreptida:

1. Possesses two pairs of legs in the fifth segment.

2. Second or posterior gonopods are al­most absent.

3. A small tail is usually present. The example is Thyropygus.

The arthropods belonging to Chilo­poda, Symphyla, Pauropoda and Diplopoda were formerly included under the class Myriapoda. The word ‘Myriapoda’ is still used to refer these animals, but is now out of taxonomic usage.

Class 5. Insecta or Hexapoda [L. in = into, sectus – cleft, cut or L. insecti = an insect Gk. hexa = six, podos, genitive of pous = a foot]:

Approx. 10,00,000 known species.

1. Size varies from 250 pm—25 cm in length.

2. Body consists of three distinct tagmata (regions)—head, thorax and abdomen.

3. Head is formed by the fusion of six segments and its appendages are a single pair of antennae, a pair of mandibles and two pairs of maxillae.

4. In adults, the thorax includes 3 seg­ments—prothorax, mesothorax and metathorax and each segment bears one pair of walking legs. Hence, called Hexapoda for the three pairs of legs.

5. In winged insects, the mesothorax and metathorax bear a pair of wings in each segment.

6. A pair of compound eyes present.

7. Paried appendages absent in the adult abdomen.

8. Respiratory organs are in the form of tracheae which extensively developed.

9. Chief excretory organs are the Mal­pighian tubules closely associated with alimentary caual.

10. Development usually pass through complicated metamorphis but in some cases it may be direct. It has two subclasses Apterygota and Pterygota.

Subclass Apterygota (Gk. a = without, pterygotos = winged):

2. Presence of terminal cerci.

Two superorder—Entognatha and Ectognatha belong to this subclass.

Super order Entognatha (Gk. entos = within gnathos = Jaw).

Labium being united with the cranium on the lateral side completely covers the mandibles and maxillae.

It includes 3 orders—Protura, Collembola and Diplura.

Order 1. Protura (Gk. proto = first, uro = tail):

1. Abdomen has twelve segments in the adult.

2. Rudimentary appendages are present on the first three abdominal segments.

3. Compound eyes and antennae are not present.

The examples are Acerentomon and Eosentomon.

Order 2. Collembola (Gk. kolla = glue, ballo = put):

1. The members are commonly called the springtails.

2. Abdomen never possesses more than six segments.

3. Eyes, Malpighian tubules and usually the tracheae are absent.

4. Last segment carries appendages for jumping.

The examples are Podura, Orchesella, Bourletiella, Isotoma and Neanura.

Order 3. Diplura [Gk. diplos – double, oura = tail]:

1. Abdomen consists of eleven segments.

2. Terminal segment of the abdomen bears cerci or forceps.

3. Malpighian tubules are usually absent. The examples are Campodea, Heterojapyx.

Super Order Ectognatha [Gk. ecto – outside):

Mandibles and maxillae are not covered by the lateral fusion of labium and cranium.

Order Thysanura (Gk. thysanos = tassel):

1. Abdomen consists of eleven segments.

2. Rudimentary appendages may occur in some abdominal segments.

3. The last segment or anal segment has two or three many-jointed anal cerci.

4. Malpighian tubules and compound eyes are usually present.

The example are Lepisma (silver fish), Machilis.

Subclass Pterygota (Gk. pterygotos = winged):

1. Adults possess wings which may be secondarily lost.

2. Excepting cerci, other appendages are absent in the abdomen.

3. Malpighian tubules are present.

4. Metamorphosis may be complete or incomplete.

This large subclass is subdivided into four sections—Paleoptera, Polyneoptera, Oligoneoptera and Paraneoptera.

Section Paleoptera:

1. At the time of rest, the wings cannot be placed parallel to the abdomen.

2. Surface of the wing is thickened only in correlation with veins.

3. Wings originate as external buds.

4. Malpighian tubules are many.

Two living orders—Ephemeroptera and Odonata are included under this section.

Order 1. Ephemeroptera:

1. The members of this order are called the mayflies.

2. Adults are aerial but larvae are aquatic.

3. Mouth parts are degenerated in adults.

4. Wings are not of same size, the hind wings are degenerated.

5. Wings appear in the last immature stage and is followed by ecdysis.

6. This is the only winged insect where ecdysis occurs after the appearance of wings.

7. Terminal part of the abdomen bears two elongated cerci and a median filament.

The examples are Ephemera, Hexagenia.

Order 2. Odonata (Gk. Odous = tooth) Dragon flies and Damsel flies:

1. All the insects are of large size.

2. Two pairs of almost equal wings.

3. At rest the wings are either held un­folded or extended laterally.

4. Rudimentary antennae are present.

5. Mouth parts are adapted for biting.

6. The eyes are very big and conspicuous.

7. Larvae are fully aquatic.

The well-known examples are Anax, Aeschna, Ischnura and Lestes.

Section Polyneoptera:

1. Wings are provided with rich supplies of veins.

2. At the time of rest, the wings are always kept folded over the abdomen.

3. Numerous Malpighian tubules are present.

This section includes nine orders which are given below:

1. Cockroaches and Preying mantids are the representatives of this order.

2. These insects usually run.

3. Mouth parts are of primitive condi­tion and used for biting.

5. Tarsi composed of five segments.

6. Eggs remain within a capsule called ootheca.

The examples are periplaneta, Mantis.

Order 2. Isoptera (Gk. isos = equal, pteron = wing):

1. The order is exemplified by the white ants or termites.

2. They exhibit polymorphism.

3. Females have much enlarged abdomen.

4. In winged forms, the wings are of same sizes and can be separated at will.

5. Each wing has a longitudinal venation and chitinised network in between.

6. Mouth parts are adapted for biting.

The examples are Cryptotermes, Ameritermes, Kalotermes, Neotermes, Glyptotermes, etc. These genera are wood termites and some of them are serious pests of rubber and tea plantations. A list of the species of the Glyptotermes (wood termites) from India is given below.

The imago of Glyptotermes is identified by head-capsule guadrate or subcircular, posterior magin round, fontanelle absent, eyes broadly oval, antennae with 11-17 segments, wings smoky brown, abdomen long, epicranial sutures present and cerci 2-jointed and short.

1. Size is extremely small.

2. They exhibit polymorphism.

3. Males are usually without wings.

4. Tarsi have two joints. The example is Zorotypus.

1. The order includes the stone flies.

5. In course of development a terrestrial stage appears which contains, only wing buds.

6. Winged forms develop from this stage and become aerial.

The examples are Perla, Isoperla.

2. Larva resembles the adult in all struc­tural details.

The example is Grylloblatta.

Order 6. Cheleutoptera:

1. Some are wingless, whereas others may have wings.

2. All the members exhibit structural features to mimic either leaves or branches of the tree.

3. Winged forms exhibit gradual appear­ance of various structures from larva to the adult.

4. But in wingless forms the larva resem­bles the adult in all structural details.

5. The eggs resemble the structures of seeds.

The examples are Carausius (stick-in- sect), Phyllium (leaf-insect).

Order 7. Orthoptera (Gk. orthos = straight, pteron = wing):

1. Grasshoppers, locusts and different crickets are the representatives of this order.

2. Structure of head resembles that of cockroach.

3. Legs of the metathoracic segment are adapted for jumping.

4. A well-developed ovipositor is present.

5. Mouth parts are of biting type.

The examples are Hieroglyphus, Tryxalis, Locusta, Schistocerca, Gryllotalpa (mole cricket).

1. The members of this order are the web-spinners.

4. Possess silk glands to form silken tun­nels for living.

Order 9. Dermaptera (Gk. derma = skin):

1. Ear-wings are the members of this order.

2. Anterior wings are short.

3. Posterior wings are papery and have radially arranged veins on its surface.

4. Posterior wings may be folded both transversely and longitudinally.

5. Mouth parts are adapted for biting.

6. Anal cerci are like forceps.

Section Oligoneoptera:

1. Metamorphosis is complete.

2. Jugal area of the wing contains only one vein.

3. Limited number of Malpighian tubules is present.

4. Wings always develop from inner wing buds.

5. Mouth parts are adapted either for biting or for sucking.

It includes following eleven orders:

Order 1. Coleoptera (Gk. Koleos = sheath):

1. The members of this order include the beetles.

2. Prothorax is freely movable.

3. Posterior pair of wing is membranous.

4. Anterior pair of wing is stiff and covers the folded posterior wing during rest.

5. Well-developed jaws are built up for biting and chewing.

6. Metamorphosis is complete.

7. Larvae may be maggot-like or cater- pillar-like.

8. No special covering is present around pupa.

The examples are Photinus, Calandra, Adalia and Dineutus.

Order 2. Megaloptera:

1. Mouth parts are adapted for biting.

2. At the margin of the wing, the longi­tudinal veins exhibit sign of bifurca­tion.

3. Metamorphosis is complete and the larvae are aquatic.

The examples are Sialis (Alder-flies), Corydalis (Dobson-flies).

Order 3. Raphidioptera:

1. The snake-flies are the members of this order.

4. Metamorphosis is complete.

5. Larvae and pupae are all terrestrial.

Order 4. Planipennia:

1. Mouth parts adapted for biting in the adult but sucking in the larva.

2. In the late larval stage, Malpighian tubules are modified to secrete silk which is used in the formation of a cocoon.

The examples are Montispa, Myrmeleon (Antlion).

5. Mecoptera (Gk. mekos = a length):

1. The members of the order are the Scorpion-flies.

2. Abdomen in male is curved upwards.

3. Head bears a beak-like prolongation and mouth parts are present at the tip of this beak.

4. Wings are membranous and all alike. The example is Panorpa.

Order 6. Trichoptera (Gk. thrix = hair):

1. Caddis-flies are the common repre­sentatives of the order.

2. Wings are hairy, membranous and are of dissimilar sizes.

3. At rest the wings remain as a roof-like peak.

4. Mandibles are generally absent.

5. Mouth parts are specialised for lick­ing.

6. Larvae are aquatic and produce silken case within which they reside.

The examples are Rhyacophilia, Mayatrichia.

Order 7. Lepidoptera (Gk. lepis = scale):

1. All the butterflies and moths belong to this order.

2. Broad and well-developed wings are enclosed by scales which are modified hairs.

3. Wings are generally oriented with varied specks of colours.

4. Maxillae in adults are modified into a spirally-coiled sucking tube.

5. Remaining mouth parts excepting the labial palps are lacking.

6. First two divisions of the thorax are fused.

7. Metamorphosis is complete.

8. Larvae possess three thoracic feet and in some cases several abdominal legs may be present.

9. Mouth parts of larvae are modified for biting.

10. Pupa is always covered with a case. The examples are Pieris, Samia, Venesa, Teinopalpus, Papilio, Parides and Bombyx.

Order 8. Diptera (Gk. dis = two):

1. Flies and mosquitoes belong to this order.

2. Metathoracic wings are modified as the halteres, which act as balancers.

3. Mesothoracic wings are welldeveloped but with sparse venation.

4. Mouth parts may be adapted for pierc­ing and sucking or only sucking.

5. Metamorphosis is complete.

The examples are Anopheles, Culex Musca, etc.

Order 9. Siphonaptera (Gk. siphon = sucker, a = without pteron = wing):

1. External parasites on warm-blooded animals.

2. Wings are absent in adults.

3. Coxae of the legs are exceedingly large.

The examples are the fleas represented by the genera, Pulex, Ctenocephalus, etc.

Order 10. Hymenoptera (Gk. hymen = membrane):

2. Two pairs of wings remain interlocked by hooks on the anterior border of hind-wing.

3. The appendages around mouth are arranged for biting, licking and suck­ing.

4. All the thoracic segments are united and the first abdominal segment is fused with it.

5. Polymorphism occurs in certain forms.

6. Metamorphosis is complete.

7. Development is complicated and lar­vae are helpless.

8. Pupa is covered by a cocoon.

The well-known examples are Apis (Honey bee), Vespa (Wasp), and Formica (Ant).

Order 11. Strepsiptera:

1. Wingless and degenerated females are endoparasites but the males are free- living and winged.

2. Anterior wings in males work as halters.

3. At rest the hind wings of male remain folded like fan.

4. Metamorphosis is complete. The example is Stylops.

Section Paraneoptera:

1. Wings develop from external wing buds.

2. Wings are usually poorly developed.

3. Very limited Malpighian tubules are present.

4. Metamorphosis varies from partially complete to fully complete condition.

5. Mouth parts are adapted for either biting or sucking.

All the forms are either parasites or pests and are included within the orders— Psocoptera, Mallophaga, Anoplura, Thysanoptera, Homoptera and Heteroptera.

1. Book-lice are the members of this order.

2. Size is extremely small.

3. Wings may or may not occur.

4. When present, the anterior pair is larger and both are membranous.

6. Mouth parts are built up for biting. The example is Psocus.

Order 2. Mallophaga (Gk. mallos = wool):

1. This order includes the bird-lice.

2. Body is dorsoventrally flattened.

5. Mouth parts are degenerated but adapted for biting.

6. Young’s resemble the adults in struc­tural details.

Order 3. Anoplura (Gk. anoplos = unarmed):

1. The order includes the sucking lice.

2. All are ectoparasites of mammals.

3. Mouth parts are adapted for piercing and sucking.

5. Body is dorsoventrally flattened. The example is Pediculus.

Order 4. Thysanoptera (Gk. thysanos = fringe):

1. All thrips are the examples of this order.

3. Mouth parts are adapted for sucking.

4. Wings may or may not be present.

5. When present the wings are slender and with elongated setae at the margin. The example is Heliothrips.

Order 5. Homoptera (Gk. homos = same):

1. Mouth parts are adapted for sucking.

2. Pronotum is rudimentary.

3. Wings are membranous and during rest are held in roof-like fashion.

The examples are Aphis, Cicada and Tachardia (Lac insect).

Order 6. Heteroptera (Gk. hetero = dis­similar):

1. Mouth parts are sucking.

3. Wings, at the time of rest, lie one over the other.

4. Mesothoracic wings are thick and its lower half is pigmented.

The examples are Cimex (Bed bug), Anasa, Leptocorisa (Rice bug), etc.

Phylogeny of Arthropods:

Whether the arthropods are monophyletic or polyphyletic has long been a debated issue. The monophyletic theory is proposed by Snodgrass (1938), Sharov (1966) and they suggest that all arthropods have evolved from a single annelid-like ancestral stock whose limbs are lobopod-like.

This lobopod- annelid gave rise to Tetracephalosomita and the most primitive arthropods— Trilobitomorpha evolved from the Tetracephalosomita, and Chelicerata and Mandibulata originated from Tetracephalosomita. Onychophora evolved from lobopod annelid-like ancestor and rep­resent an early lateral branch of the evolu­tionary line (Fig. 18.120).

The monophyletic theory is based on the segmentation be­tween the annelids and arthropods, and the prostomium and pygidium of the annelids correspond to the acron and telson of arthro­pods.

A number of anatomical features, such as chitinous cuticle, haemocoel, dorsal blood vessel, segmental jointed appendages and centrolecithal eggs constitute the basis for the view that the arthropods are mono­phyletic (Anderson, 1998).

The polyphyletic theory is proposed by Tiegs and Manton (1958).

They suggested that there were two ancestors of arthro­pods:

(ii) Proto- annelids (Fig. 18.121).

(i) The lobopod-annelid is considered to be the ancestor of Onychophora which was originated in the Pre-Cambrian period about 525 million years ago. The group Uniramia which includes chilopodes, diplopodes and insects is thought that they have evolved from the base of onychophores.

(ii) The protoannelid ancestor is consi­dered as the primitive annelids from which the most primitive extinct arthropod group- Trilobitomorpha arose, from which two ex­tant groups—Crustacea and Chelicerata evolved by two separate evolutionary lines. Manton (1973), Ruppert and Barnes (1994), support the polyphyletic theory.


A unique feature of animals in the arthropod phylum is the presence of a segmented body and fusion of sets of segments that give rise to functional body regions called tagma. Tagma may be in the form of a head, thorax, and abdomen, or a cephalothorax and abdomen, or a head and trunk. A central cavity, called the hemocoel (or blood cavity), is present, and the open circulatory system is regulated by a tubular or single-chambered heart. Respiratory systems vary depending on the group of arthropod: insects and myriapods use a series of tubes (tracheae) that branch through the body, open to the outside through openings called spiracles, and perform gas exchange directly between the cells and air in the tracheae, whereas aquatic crustaceans utilize gills, terrestrial chelicerates employ book lungs, and aquatic chelicerates use book gills (Figure 2).

Figure 2. The book lungs of (a) arachnids are made up of alternating air pockets and hemocoel tissue shaped like a stack of books. The book gills of (b) crustaceans are similar to book lungs but are external so that gas exchange can occur with the surrounding water. (credit a: modification of work by Ryan Wilson based on original work by John Henry Comstock credit b: modification of work by Angel Schatz)

The book lungs of arachnids (scorpions, spiders, ticks and mites) contain a vertical stack of hemocoel wall tissue that somewhat resembles the pages of a book. Between each of the “pages” of tissue is an air space. This allows both sides of the tissue to be in contact with the air at all times, greatly increasing the efficiency of gas exchange. The gills of crustaceans are filamentous structures that exchange gases with the surrounding water. Groups of arthropods also differ in the organs used for excretion, with crustaceans possessing green glands and insects using Malpighian tubules, which work in conjunction with the hindgut to reabsorb water while ridding the body of nitrogenous waste. The cuticle is the covering of an arthropod. It is made up of two layers: the epicuticle, which is a thin, waxy water-resistant outer layer containing no chitin, and the layer beneath it, the chitinous procuticle. Chitin is a tough, flexible polysaccharide. In order to grow, the arthropod must shed the exoskeleton during a process called ecdysis (“to strip off”) this is a cumbersome method of growth, and during this time, the animal is vulnerable to predation.

What is this arthropod? - Biology

Many bugs, known as arthropods, make their home in the soil. They get their name from their jointed (arthros) legs (podos). Arthropods are invertebrates, that is, they have no backbone, and rely instead on an external covering called an exoskeleton.

Arthropods range in size from microscopic to several inches in length. They include insects, such as springtails, beetles, and ants crustaceans such as sowbugs arachnids such as spiders and mites myriapods, such as centipedes and millipedes and scorpions.

Nearly every soil is home to many different arthropod species. Certain row-crop soils contain several dozen species of arthropods in a square mile. Several thousand different species may live in a square mile of forest soil.

Arthropods can be grouped as shredders, predators, herbivores, and fungal-feeders, based on their functions in soil. Most soil-dwelling arthropods eat fungi, worms, or other arthropods. Root-feeders and dead-plant shredders are less abundant. As they feed, arthropods aerate and mix the soil, regulate the population size of other soil organisms, and shred organic material.


Many large arthropods frequently seen on the soil surface are shredders. Shredders chew up dead plant matter as they eat bacteria and fungi on the surface of the plant matter. The most abundant shredders are millipedes and sowbugs, as well as termites, certain mites, and roaches. In agricultural soils, shredders can become pests by feeding on live roots if sufficient dead plant material is not present.


Predators and micropredators can be either generalists, feeding on many different prey types, or specialists, hunting only a single prey type. Predators include centipedes, spiders, ground-beetles, scorpions, skunk-spiders, pseudoscorpions, ants, and some mites. Many predators eat crop pests, and some, such as beetles and parasitic wasps, have been developed for use as commercial biocontrols.


Numerous root-feeding insects, such as cicadas, mole-crickets, and anthomyiid flies (root-maggots), live part of all of their life in the soil. Some herbivores, including rootworms and symphylans, can be crop pests where they occur in large numbers, feeding on roots or other plant parts.

Figure 14: The symphylan, a relative of the centipede, feeds on plant roots and can become a major crop pest if its population is not controlled by other organisms.
Credit: Ken Gray Collection, Department of Entomology, Oregon State University, Corvallis.


Arthropods that graze on fungi (and to some extent bacteria) include most springtails, some mites, and silverfish. They scrape and consume bacteria and fungi off root surfaces. A large fraction of the nutrients available to plants is a result of microbial-grazing and nutrient release by fauna.


If you would like to see what kind of organisms are in your soil, you can easily make a pitfall trap to catch large arthropods, and a Burlese funnel to catch small arthropods.

Make a pitfall trap by sinking a pint- or quart-sized container (such as a yogurt cup) into the ground so the rim is level with the soil surface. If desired, fashion a roof over the cup to keep the rain out, and add 1/2 of an inch of non-hazardous antifreeze to the cup to preserve the creatures and prevent them from eating one another. Leave in place for a week and wait for soil organisms to fall into the trap.

To make a Burlese funnel, set a piece of 1/4 inch rigid wire screen in the bottom of a funnel to support the soil. (A funnel can be made by cutting the bottom off a plastic soda bottle.) Half fill the funnel with soil, and suspend it over a cup with a bit of anti-freeze or ethyl alcohol in the bottom as a preservative.

Suspend a light bulb about 4 inches over the soil to drive the organisms out of the soil and into the cup. Leave the light bulb on for about 3 days to dry out the soil. Then pour the alcohol into a shallow dish and use a magnifying glass to examine the organisms.


Although the plant feeders can become pests, most arthropods perform beneficial functions in the soil-plant system.

Shred organic material. Arthropods increase the surface area accessible to microbial attack by shredding dead plant residue and burrowing into coarse woody debris. Without shredders, a bacterium in leaf litter would be like a person in a pantry without a can-opener – eating would be a very slow process. The shredders act like can-openers and greatly increase the rate of decomposition. Arthropods ingest decaying plant material to eat the bacteria and fungi on the surface of the organic material.

Stimulate microbial activity. As arthropods graze on bacteria and fungi, they stimulate the growth of mycorrhizae and other fungi, and the decomposition of organic matter. If grazer populations get too dense the opposite effect can occur – populations of bacteria and fungi will decline. Predatory arthropods are important to keep grazer populations under control and to prevent them from over-grazing microbes.

Mix microbes with their food. From a bacterium’s point-of-view, just a fraction of a millimeter is infinitely far away. Bacteria have limited mobility in soil and a competitor is likely to be closer to a nutrient treasure. Arthropods help out by distributing nutrients through the soil, and by carrying bacteria on their exoskeleton and through their digestive system. By more thoroughly mixing microbes with their food, arthropods enhance organic matter decomposition.

Mineralize plant nutrients. As they graze, arthropods mineralize some of the nutrients in bacteria and fungi, and excrete nutrients in plant-available forms.

Enhance soil aggregation. In most forested and grassland soils, every particle in the upper several inches of soil has been through the gut of numerous soil fauna. Each time soil passes through another arthropod or earthworm, it is thoroughly mixed with organic matter and mucus and deposited as fecal pellets. Fecal pellets are a highly concentrated nutrient resource, and are a mixture of the organic and inorganic substances required for growth of bacteria and fungi. In many soils, aggregates between 1/10,000 and 1/10 of an inch (0.0025mm and 2.5mm) are actually fecal pellets.

Burrow. Relatively few arthropod species burrow through the soil. Yet, within any soil community, burrowing arthropods and earthworms exert an enormous influence on the composition of the total fauna by shaping habitat. Burrowing changes the physical properties of soil, including porosity, water-infiltration rate, and bulk density.

Stimulate the succession of species. A dizzying array of natural bio-organic chemicals permeates the soil. Complete digestion of these chemicals requires a series of many types of bacteria, fungi, and other organisms with different enzymes. At any time, only a small subset of species is metabolically active – only those capable of using the resources currently available. Soil arthropods consume the dominant organisms and permit other species to move in and take their place, thus facilitating the progressive breakdown of soil organic matter.

Control pests. Some arthropods can be damaging to crop yields, but many others that are present in all soils eat or compete with various root- and foliage-feeders. Some (the specialists) feed on only a single type of prey species. Other arthropods (the generalists), such as many species of centipedes, spiders, ground-beetles, rove-beetles, and gamasid mites, feed on a broad range of prey. Where a healthy population of generalist predators is present, they will be available to deal with a variety of pest outbreaks. A population of predators can only be maintained between pest outbreaks if there is a constant source of non-pest prey to eat. That is, there must be a healthy and diverse food web.

A fundamental dilemma in pest control is that tillage and insecticide application have enormous effects on non- target species in the food web. Intense land use (especially monoculture, tillage, and pesticides) depletes soil diversity. As total soil diversity declines, predator populations drop sharply and the possibility for subsequent pest outbreaks increases.


The abundance and diversity of soil fauna diminishes significantly with soil depth. The great majority of all soil species are confined to the top three inches. Most of these creatures have limited mobility, and are probably capable of “cryptobiosis,” a state of “suspended animation” that helps them survive extremes of temperature, wetness, or dryness that would otherwise be lethal.

As a general rule, larger species are active on the soil surface, seeking temporary refuge under vegetation, plant residue, wood, or rocks. Many of these arthropods commute daily to forage within herbaceous vegetation above, or even high in the canopy of trees. (For instance, one of these tree-climbers is the caterpillar-searcher used by foresters to control gypsy moth). Some large species capable of true burrowing live within the deeper layers of the soil.

Below about two inches in the soil, fauna are generally small – 1/250 to 1/10 of an inch. (Twenty-five of the smallest of these would fit in a period on this page.) These species are usually blind and lack prominent coloration. They are capable of squeezing through minute pore spaces and along root channels. Sub-surface soil dwellers are associated primarily with the rhizosphere (the soil volume immediately adjacent to roots).


A single square yard of soil will contain 500 to 200,000 individual arthropods, depending upon the soil type, plant community, and management system. Despite these large numbers, the biomass of arthropods in soil is far less than that of protozoa and nematodes.

In most environments, the most abundant soil dwellers are springtails and mites, though ants and termites predominate in certain situations, especially in desert and tropical soils. The largest number of arthropods are in natural plant communities with few earthworms (such as conifer forests). Natural communities with numerous earthworms (such as grassland soils) have the fewest arthropods. Apparently, earthworms out-compete arthropods, perhaps by excessively reworking their habitat or eating them incidentally. However, within pastures and farm lands arthropod numbers and diversity are generally thought to increase as earthworm populations rise. Burrowing earthworms probably create habitat space for arthropods in agricultural soils.

BUG BIOGRAPHY: Springtails
Springtails are the most abundant arthropods in many agricultural and rangeland soils. populations of tens of thousands per square yard are frequent. When foraging, springtails walk with 3 pairs of legs like most insects, and hold their tail tightly tucked under the belly. If attacked by a predator, body fluid rushes into the tail base, forcing the tail to slam down and catapult the springtail as much as a yard away. Springtails have been shown to be beneficial to crop plants by releasing nutrients and by feeding upon diseases caused by fungi.


The word arthropod comes from the Greek ἄρθρον árthron, "joint", and πούς pous (gen. podos (ποδός)), i.e. "foot" or "leg", which together mean "jointed leg". [4] The designation "Arthropoda" was coined in 1848 by the German physiologist and zoologist Karl Theodor Ernst von Siebold (1804–1885). [5] [6]

Arthropods are invertebrates with segmented bodies and jointed limbs. [7] The exoskeleton or cuticles consists of chitin, a polymer of glucosamine. [8] The cuticle of many crustaceans, beetle mites, and millipedes (except for bristly millipedes) is also biomineralized with calcium carbonate. Calcification of the endosternite, an internal structure used for muscle attachments, also occur in some opiliones. [9]

Diversity Edit

Estimates of the number of arthropod species vary between 1,170,000 and 5 to 10 million and account for over 80 percent of all known living animal species. [10] [11] The number of species remains difficult to determine. This is due to the census modeling assumptions projected onto other regions in order to scale up from counts at specific locations applied to the whole world. A study in 1992 estimated that there were 500,000 species of animals and plants in Costa Rica alone, of which 365,000 were arthropods. [12]

They are important members of marine, freshwater, land and air ecosystems, and are one of only two major animal groups that have adapted to life in dry environments the other is amniotes, whose living members are reptiles, birds and mammals. [13] One arthropod sub-group, insects, is the most species-rich member of all ecological guilds in land and freshwater environments. [12] The lightest insects weigh less than 25 micrograms (millionths of a gram), [14] while the heaviest weigh over 70 grams ( 2 + 1 ⁄ 2 oz). [15] Some living crustaceans are much larger for example, the legs of the Japanese spider crab may span up to 4 metres (13 ft), [14] with the heaviest of all living arthropods being the American lobster, topping out at over 20 kg (44 lbs).

Segmentation Edit

The embryos of all arthropods are segmented, built from a series of repeated modules. The last common ancestor of living arthropods probably consisted of a series of undifferentiated segments, each with a pair of appendages that functioned as limbs. However, all known living and fossil arthropods have grouped segments into tagmata in which segments and their limbs are specialized in various ways. [13]

The three-part appearance of many insect bodies and the two-part appearance of spiders is a result of this grouping [16] in fact there are no external signs of segmentation in mites. [13] Arthropods also have two body elements that are not part of this serially repeated pattern of segments, an acron at the front, ahead of the mouth, and a telson at the rear, behind the anus. The eyes are mounted on the acron. [13]

Originally it seems that each appendage-bearing segment had two separate pairs of appendages: an upper and a lower pair. These would later fuse into a single pair of biramous appendages, with the upper branch acting as a gill while the lower branch was used for locomotion. [17] In some segments of all known arthropods the appendages have been modified, for example to form gills, mouth-parts, antennae for collecting information, [16] or claws for grasping [18] arthropods are "like Swiss Army knives, each equipped with a unique set of specialized tools." [13] In many arthropods, appendages have vanished from some regions of the body it is particularly common for abdominal appendages to have disappeared or be highly modified. [13]

The most conspicuous specialization of segments is in the head. The four major groups of arthropods – Chelicerata (includes spiders and scorpions), Crustacea (shrimps, lobsters, crabs, etc.), Tracheata (arthropods that breathe via channels into their bodies includes insects and myriapods), and the extinct trilobites – have heads formed of various combinations of segments, with appendages that are missing or specialized in different ways. [13] In addition, some extinct arthropods, such as Marrella, belong to none of these groups, as their heads are formed by their own particular combinations of segments and specialized appendages. [19]

Working out the evolutionary stages by which all these different combinations could have appeared is so difficult that it has long been known as "the arthropod head problem". [20] In 1960, R. E. Snodgrass even hoped it would not be solved, as he found trying to work out solutions to be fun. [Note 1]

Exoskeleton Edit

Arthropod exoskeletons are made of cuticle, a non-cellular material secreted by the epidermis. [13] Their cuticles vary in the details of their structure, but generally consist of three main layers: the epicuticle, a thin outer waxy coat that moisture-proofs the other layers and gives them some protection the exocuticle, which consists of chitin and chemically hardened proteins and the endocuticle, which consists of chitin and unhardened proteins. The exocuticle and endocuticle together are known as the procuticle. [22] Each body segment and limb section is encased in hardened cuticle. The joints between body segments and between limb sections are covered by flexible cuticle. [13]

The exoskeletons of most aquatic crustaceans are biomineralized with calcium carbonate extracted from the water. Some terrestrial crustaceans have developed means of storing the mineral, since on land they cannot rely on a steady supply of dissolved calcium carbonate. [23] Biomineralization generally affects the exocuticle and the outer part of the endocuticle. [22] Two recent hypotheses about the evolution of biomineralization in arthropods and other groups of animals propose that it provides tougher defensive armor, [24] and that it allows animals to grow larger and stronger by providing more rigid skeletons [25] and in either case a mineral-organic composite exoskeleton is cheaper to build than an all-organic one of comparable strength. [25] [26]

The cuticle may have setae (bristles) growing from special cells in the epidermis. Setae are as varied in form and function as appendages. For example, they are often used as sensors to detect air or water currents, or contact with objects aquatic arthropods use feather-like setae to increase the surface area of swimming appendages and to filter food particles out of water aquatic insects, which are air-breathers, use thick felt-like coats of setae to trap air, extending the time they can spend under water heavy, rigid setae serve as defensive spines. [13]

Although all arthropods use muscles attached to the inside of the exoskeleton to flex their limbs, some still use hydraulic pressure to extend them, a system inherited from their pre-arthropod ancestors [27] for example, all spiders extend their legs hydraulically and can generate pressures up to eight times their resting level. [28]

Moulting Edit

The exoskeleton cannot stretch and thus restricts growth. Arthropods, therefore, replace their exoskeletons by undergoing ecdysis (moulting), or shedding the old exoskeleton after growing a new one that is not yet hardened. Moulting cycles run nearly continuously until an arthropod reaches full size. [29]

The developmental stages between each moult (ecdysis) until sexual maturity is reached is called an instar. Differences between instars can often be seen in altered body proportions, colors, patterns, changes in the number of body segments or head width. After moulting, i.e. shedding their exoskeleton, the juvenile arthropods continue in their life cycle until they either pupate or moult again.

In the initial phase of moulting, the animal stops feeding and its epidermis releases moulting fluid, a mixture of enzymes that digests the endocuticle and thus detaches the old cuticle. This phase begins when the epidermis has secreted a new epicuticle to protect it from the enzymes, and the epidermis secretes the new exocuticle while the old cuticle is detaching. When this stage is complete, the animal makes its body swell by taking in a large quantity of water or air, and this makes the old cuticle split along predefined weaknesses where the old exocuticle was thinnest. It commonly takes several minutes for the animal to struggle out of the old cuticle. At this point, the new one is wrinkled and so soft that the animal cannot support itself and finds it very difficult to move, and the new endocuticle has not yet formed. The animal continues to pump itself up to stretch the new cuticle as much as possible, then hardens the new exocuticle and eliminates the excess air or water. By the end of this phase, the new endocuticle has formed. Many arthropods then eat the discarded cuticle to reclaim its materials. [29]

Because arthropods are unprotected and nearly immobilized until the new cuticle has hardened, they are in danger both of being trapped in the old cuticle and of being attacked by predators. Moulting may be responsible for 80 to 90% of all arthropod deaths. [29]

Internal organs Edit

Arthropod bodies are also segmented internally, and the nervous, muscular, circulatory, and excretory systems have repeated components. [13] Arthropods come from a lineage of animals that have a coelom, a membrane-lined cavity between the gut and the body wall that accommodates the internal organs. The strong, segmented limbs of arthropods eliminate the need for one of the coelom's main ancestral functions, as a hydrostatic skeleton, which muscles compress in order to change the animal's shape and thus enable it to move. Hence the coelom of the arthropod is reduced to small areas around the reproductive and excretory systems. Its place is largely taken by a hemocoel, a cavity that runs most of the length of the body and through which blood flows. [30]

Respiration and circulation Edit

Arthropods have open circulatory systems, although most have a few short, open-ended arteries. In chelicerates and crustaceans, the blood carries oxygen to the tissues, while hexapods use a separate system of tracheae. Many crustaceans, but few chelicerates and tracheates, use respiratory pigments to assist oxygen transport. The most common respiratory pigment in arthropods is copper-based hemocyanin this is used by many crustaceans and a few centipedes. A few crustaceans and insects use iron-based hemoglobin, the respiratory pigment used by vertebrates. As with other invertebrates, the respiratory pigments of those arthropods that have them are generally dissolved in the blood and rarely enclosed in corpuscles as they are in vertebrates. [30]

The heart is typically a muscular tube that runs just under the back and for most of the length of the hemocoel. It contracts in ripples that run from rear to front, pushing blood forwards. Sections not being squeezed by the heart muscle are expanded either by elastic ligaments or by small muscles, in either case connecting the heart to the body wall. Along the heart run a series of paired ostia, non-return valves that allow blood to enter the heart but prevent it from leaving before it reaches the front. [30]

Arthropods have a wide variety of respiratory systems. Small species often do not have any, since their high ratio of surface area to volume enables simple diffusion through the body surface to supply enough oxygen. Crustacea usually have gills that are modified appendages. Many arachnids have book lungs. [31] Tracheae, systems of branching tunnels that run from the openings in the body walls, deliver oxygen directly to individual cells in many insects, myriapods and arachnids. [32]

Nervous system Edit

Living arthropods have paired main nerve cords running along their bodies below the gut, and in each segment the cords form a pair of ganglia from which sensory and motor nerves run to other parts of the segment. Although the pairs of ganglia in each segment often appear physically fused, they are connected by commissures (relatively large bundles of nerves), which give arthropod nervous systems a characteristic "ladder-like" appearance. The brain is in the head, encircling and mainly above the esophagus. It consists of the fused ganglia of the acron and one or two of the foremost segments that form the head – a total of three pairs of ganglia in most arthropods, but only two in chelicerates, which do not have antennae or the ganglion connected to them. The ganglia of other head segments are often close to the brain and function as part of it. In insects these other head ganglia combine into a pair of subesophageal ganglia, under and behind the esophagus. Spiders take this process a step further, as all the segmental ganglia are incorporated into the subesophageal ganglia, which occupy most of the space in the cephalothorax (front "super-segment"). [33]

Excretory system Edit

There are two different types of arthropod excretory systems. In aquatic arthropods, the end-product of biochemical reactions that metabolise nitrogen is ammonia, which is so toxic that it needs to be diluted as much as possible with water. The ammonia is then eliminated via any permeable membrane, mainly through the gills. [31] All crustaceans use this system, and its high consumption of water may be responsible for the relative lack of success of crustaceans as land animals. [34] Various groups of terrestrial arthropods have independently developed a different system: the end-product of nitrogen metabolism is uric acid, which can be excreted as dry material the Malpighian tubule system filters the uric acid and other nitrogenous waste out of the blood in the hemocoel, and dumps these materials into the hindgut, from which they are expelled as feces. [34] Most aquatic arthropods and some terrestrial ones also have organs called nephridia ("little kidneys"), which extract other wastes for excretion as urine. [34]

Senses Edit

The stiff cuticles of arthropods would block out information about the outside world, except that they are penetrated by many sensors or connections from sensors to the nervous system. In fact, arthropods have modified their cuticles into elaborate arrays of sensors. Various touch sensors, mostly setae, respond to different levels of force, from strong contact to very weak air currents. Chemical sensors provide equivalents of taste and smell, often by means of setae. Pressure sensors often take the form of membranes that function as eardrums, but are connected directly to nerves rather than to auditory ossicles. The antennae of most hexapods include sensor packages that monitor humidity, moisture and temperature. [35]

Optical Edit

Most arthropods have sophisticated visual systems that include one or more usually both of compound eyes and pigment-cup ocelli ("little eyes"). In most cases ocelli are only capable of detecting the direction from which light is coming, using the shadow cast by the walls of the cup. However, the main eyes of spiders are pigment-cup ocelli that are capable of forming images, [35] and those of jumping spiders can rotate to track prey. [36]

Compound eyes consist of fifteen to several thousand independent ommatidia, columns that are usually hexagonal in cross section. Each ommatidium is an independent sensor, with its own light-sensitive cells and often with its own lens and cornea. [35] Compound eyes have a wide field of view, and can detect fast movement and, in some cases, the polarization of light. [37] On the other hand, the relatively large size of ommatidia makes the images rather coarse, and compound eyes are shorter-sighted than those of birds and mammals – although this is not a severe disadvantage, as objects and events within 20 cm (8 in) are most important to most arthropods. [35] Several arthropods have color vision, and that of some insects has been studied in detail for example, the ommatidia of bees contain receptors for both green and ultra-violet. [35]

Most arthropods lack balance and acceleration sensors, and rely on their eyes to tell them which way is up. The self-righting behavior of cockroaches is triggered when pressure sensors on the underside of the feet report no pressure. However, many malacostracan crustaceans have statocysts, which provide the same sort of information as the balance and motion sensors of the vertebrate inner ear. [35]

The proprioceptors of arthropods, sensors that report the force exerted by muscles and the degree of bending in the body and joints, are well understood. However, little is known about what other internal sensors arthropods may have. [35]

Olfaction Edit

A few arthropods, such as barnacles, are hermaphroditic, that is, each can have the organs of both sexes. However, individuals of most species remain of one sex their entire lives. [38] A few species of insects and crustaceans can reproduce by parthenogenesis, especially if conditions favor a "population explosion". However, most arthropods rely on sexual reproduction, and parthenogenetic species often revert to sexual reproduction when conditions become less favorable. [39] Aquatic arthropods may breed by external fertilization, as for example frogs do, or by internal fertilization, where the ova remain in the female's body and the sperm must somehow be inserted. All known terrestrial arthropods use internal fertilization. Opiliones (harvestmen), millipedes, and some crustaceans use modified appendages such as gonopods or penises to transfer the sperm directly to the female. However, most male terrestrial arthropods produce spermatophores, waterproof packets of sperm, which the females take into their bodies. A few such species rely on females to find spermatophores that have already been deposited on the ground, but in most cases males only deposit spermatophores when complex courtship rituals look likely to be successful. [38]

Most arthropods lay eggs, [38] but scorpions are ovoviparous: they produce live young after the eggs have hatched inside the mother, and are noted for prolonged maternal care. [40] Newly born arthropods have diverse forms, and insects alone cover the range of extremes. Some hatch as apparently miniature adults (direct development), and in some cases, such as silverfish, the hatchlings do not feed and may be helpless until after their first moult. Many insects hatch as grubs or caterpillars, which do not have segmented limbs or hardened cuticles, and metamorphose into adult forms by entering an inactive phase in which the larval tissues are broken down and re-used to build the adult body. [41] Dragonfly larvae have the typical cuticles and jointed limbs of arthropods but are flightless water-breathers with extendable jaws. [42] Crustaceans commonly hatch as tiny nauplius larvae that have only three segments and pairs of appendages. [38]

Last common ancestor Edit

The last common ancestor of all arthropods is reconstructed as a modular organism with each module covered by its own sclerite (armor plate) and bearing a pair of biramous limbs. [43] However, whether the ancestral limb was uniramous or biramous is far from a settled debate. This Ur-arthropod had a ventral mouth, pre-oral antennae and dorsal eyes at the front of the body. It was assumed it was a non-discriminatory sediment feeder, processing whatever sediment came its way for food, [43] but fossil findings hints that the last common ancestor of both arthropods and priapulida shared the same specialized mouth apparatus a circular mouth with rings of teeth used for capturing prey and was therefore carnivorous. [44]

Fossil record Edit

It has been proposed that the Ediacaran animals Parvancorina and Spriggina, from around 555 million years ago , were arthropods. [45] [46] [47] Small arthropods with bivalve-like shells have been found in Early Cambrian fossil beds dating 541 to 539 million years ago in China and Australia. [48] [49] [50] [51] The earliest Cambrian trilobite fossils are about 530 million years old, but the class was already quite diverse and worldwide, suggesting that they had been around for quite some time. [52] Re-examination in the 1970s of the Burgess Shale fossils from about 505 million years ago identified many arthropods, some of which could not be assigned to any of the well-known groups, and thus intensified the debate about the Cambrian explosion. [53] [54] [55] A fossil of Marrella from the Burgess Shale has provided the earliest clear evidence of moulting. [56]

The earliest fossil crustaceans date from about 511 million years ago in the Cambrian, [58] and fossil shrimp from about 500 million years ago apparently formed a tight-knit procession across the seabed. [59] Crustacean fossils are common from the Ordovician period onwards. [60] They have remained almost entirely aquatic, possibly because they never developed excretory systems that conserve water. [34] In 2020 scientists announced the discovery of Kylinxia, a five-eyed

5 cm long shrimp-like animal living 518 Mya that – with multiple distinctive features – appears to be a key ‘missing link’ of the evolution from Anomalocaris to true arthropods and could be at the evolutionary root of true arthropods. [57] [61]

Arthropods provide the earliest identifiable fossils of land animals, from about 419 million years ago in the Late Silurian, [31] and terrestrial tracks from about 450 million years ago appear to have been made by arthropods. [62] Arthropods were well pre-adapted to colonize land, because their existing jointed exoskeletons provided protection against desiccation, support against gravity and a means of locomotion that was not dependent on water. [63] Around the same time the aquatic, scorpion-like eurypterids became the largest ever arthropods, some as long as 2.5 m (8 ft 2 in). [64]

The oldest known arachnid is the trigonotarbid Palaeotarbus jerami, from about 420 million years ago in the Silurian period. [65] [Note 2] Attercopus fimbriunguis, from 386 million years ago in the Devonian period, bears the earliest known silk-producing spigots, but its lack of spinnerets means it was not one of the true spiders, [67] which first appear in the Late Carboniferous over 299 million years ago . [68] The Jurassic and Cretaceous periods provide a large number of fossil spiders, including representatives of many modern families. [69] Fossils of aquatic scorpions with gills appear in the Silurian and Devonian periods, and the earliest fossil of an air-breathing scorpion with book lungs dates from the Early Carboniferous period. [70]

The oldest definitive insect fossil is the Devonian Rhyniognatha hirsti, dated at 396 to 407 million years ago , but its mandibles are of a type found only in winged insects, which suggests that the earliest insects appeared in the Silurian period. [71] The Mazon Creek lagerstätten from the Late Carboniferous, about 300 million years ago , include about 200 species, some gigantic by modern standards, and indicate that insects had occupied their main modern ecological niches as herbivores, detritivores and insectivores. Social termites and ants first appear in the Early Cretaceous, and advanced social bees have been found in Late Cretaceous rocks but did not become abundant until the Middle Cenozoic. [72]

Evolutionary family tree Edit

From 1952 to 1977, zoologist Sidnie Manton and others argued that arthropods are polyphyletic, in other words, that they do not share a common ancestor that was itself an arthropod. Instead, they proposed that three separate groups of "arthropods" evolved separately from common worm-like ancestors: the chelicerates, including spiders and scorpions the crustaceans and the uniramia, consisting of onychophorans, myriapods and hexapods. These arguments usually bypassed trilobites, as the evolutionary relationships of this class were unclear. Proponents of polyphyly argued the following: that the similarities between these groups are the results of convergent evolution, as natural consequences of having rigid, segmented exoskeletons that the three groups use different chemical means of hardening the cuticle that there were significant differences in the construction of their compound eyes that it is hard to see how such different configurations of segments and appendages in the head could have evolved from the same ancestor and that crustaceans have biramous limbs with separate gill and leg branches, while the other two groups have uniramous limbs in which the single branch serves as a leg. [74]

anomalocarid-like taxa,
including modern tardigrades as
well as extinct animals like
Kerygmachela and Opabinia

including living groups and
extinct forms such as trilobites

Further analysis and discoveries in the 1990s reversed this view, and led to acceptance that arthropods are monophyletic, in other words they do share a common ancestor that was itself an arthropod. [75] [76] For example, Graham Budd's analyses of Kerygmachela in 1993 and of Opabinia in 1996 convinced him that these animals were similar to onychophorans and to various Early Cambrian "lobopods", and he presented an "evolutionary family tree" that showed these as "aunts" and "cousins" of all arthropods. [73] [77] These changes made the scope of the term "arthropod" unclear, and Claus Nielsen proposed that the wider group should be labelled "Panarthropoda" ("all the arthropods") while the animals with jointed limbs and hardened cuticles should be called "Euarthropoda" ("true arthropods"). [78]

A contrary view was presented in 2003, when Jan Bergström and Xian-Guang Hou argued that, if arthropods were a "sister-group" to any of the anomalocarids, they must have lost and then re-evolved features that were well-developed in the anomalocarids. The earliest known arthropods ate mud in order to extract food particles from it, and possessed variable numbers of segments with unspecialized appendages that functioned as both gills and legs. Anomalocarids were, by the standards of the time, huge and sophisticated predators with specialized mouths and grasping appendages, fixed numbers of segments some of which were specialized, tail fins, and gills that were very different from those of arthropods. This reasoning implies that Parapeytoia, which has legs and a backward-pointing mouth like that of the earliest arthropods, is a more credible closest relative of arthropods than is Anomalocaris. [79] In 2006, they suggested that arthropods were more closely related to lobopods and tardigrades than to anomalocarids. [80] In 2014, research indicated that tardigrades were more closely related to arthropods than velvet worms. [81]

Higher up the "family tree", the Annelida have traditionally been considered the closest relatives of the Panarthropoda, since both groups have segmented bodies, and the combination of these groups was labelled Articulata. There had been competing proposals that arthropods were closely related to other groups such as nematodes, priapulids and tardigrades, but these remained minority views because it was difficult to specify in detail the relationships between these groups.

In the 1990s, molecular phylogenetic analyses of DNA sequences produced a coherent scheme showing arthropods as members of a superphylum labelled Ecdysozoa ("animals that moult"), which contained nematodes, priapulids and tardigrades but excluded annelids. This was backed up by studies of the anatomy and development of these animals, which showed that many of the features that supported the Articulata hypothesis showed significant differences between annelids and the earliest Panarthropods in their details, and some were hardly present at all in arthropods. This hypothesis groups annelids with molluscs and brachiopods in another superphylum, Lophotrochozoa.

If the Ecdysozoa hypothesis is correct, then segmentation of arthropods and annelids either has evolved convergently or has been inherited from a much older ancestor and subsequently lost in several other lineages, such as the non-arthropod members of the Ecdysozoa. [84] [82]

Arthropods belong to phylum Euarthropoda. [3] [85] The phylum is sometimes called Arthropoda, but strictly this term denotes a (putative - see Tactopoda) clade that also encompasses the phylum Onychophora. [1]

Euarthropoda is typically subdivided into five subphyla, of which one is extinct: [86]

  1. Trilobites were an extinct group of formerly numerous marine animals that disappeared in the Permian–Triassic extinction event, though they were in decline prior to this killing blow, having been reduced to one order in the Late Devonian extinction.
  2. Chelicerates include horseshoe crabs, spiders, mites, scorpions and related organisms characterized by the presence of chelicerae, appendages just above/in front of the mouthparts. Chelicerae appear in scorpions and horseshoe crabs as tiny claws that they use in feeding, but those of spiders have developed as fangs that inject venom.
  3. Myriapods comprise millipedes, centipedes and their relatives, characterized by having numerous body segments each of which bearing one or two pairs of legs (or in a few cases being legless). They are sometimes grouped with the hexapods.
  4. Crustaceans are primarily aquatic (a notable exception being woodlice, which is purely terrestrial) and are characterized by having biramous appendages. They include lobsters, crabs, barnacles, crayfish, shrimp and many others.
  5. Hexapods comprise insects and three small orders of insect-like animals with six thoracic legs. They are sometimes grouped with the myriapods, in a group called Uniramia, though genetic evidence tends to support a closer relationship between hexapods and crustaceans.

Aside from these major groups, there are also a number of fossil forms, mostly from the early Cambrian period, which are difficult to place taxonomically, either from lack of obvious affinity to any of the main groups or from clear affinity to several of them. Marrella was the first one to be recognized as significantly different from the well-known groups. [19]

The phylogeny of the major extant arthropod groups has been an area of considerable interest and dispute. [87] Recent studies strongly suggest that Crustacea, as traditionally defined, is paraphyletic, with Hexapoda having evolved from within it, [88] [89] so that Crustacea and Hexapoda form a clade, Pancrustacea. The position of Myriapoda, Chelicerata and Pancrustacea remains unclear as of April 2012 [update] . In some studies, Myriapoda is grouped with Chelicerata (forming Myriochelata) [90] [91] in other studies, Myriapoda is grouped with Pancrustacea (forming Mandibulata), [88] or Myriapoda may be sister to Chelicerata plus Pancrustacea. [89]

The placement of the extinct trilobites is also a frequent subject of dispute. [92] One of the newer hypotheses is that the chelicerae have originated from the same pair of appendages that evolved into antennae in the ancestors of Mandibulata, which would place trilobites, which had antennae, closer to Mandibulata than Chelicerata. [93]

Since the International Code of Zoological Nomenclature recognises no priority above the rank of family, many of the higher-level groups can be referred to by a variety of different names. [94] [ better source needed ]

Crustaceans such as crabs, lobsters, crayfish, shrimp, and prawns have long been part of human cuisine, and are now raised commercially. [95] Insects and their grubs are at least as nutritious as meat, and are eaten both raw and cooked in many cultures, though not most European, Hindu, and Islamic cultures. [96] [97] Cooked tarantulas are considered a delicacy in Cambodia, [98] [99] [100] and by the Piaroa Indians of southern Venezuela, after the highly irritant hairs – the spider's main defense system – are removed. [101] Humans also unintentionally eat arthropods in other foods, [102] and food safety regulations lay down acceptable contamination levels for different kinds of food material. [Note 3] [Note 4] The intentional cultivation of arthropods and other small animals for human food, referred to as minilivestock, is now emerging in animal husbandry as an ecologically sound concept. [106] Commercial butterfly breeding provides Lepidoptera stock to butterfly conservatories, educational exhibits, schools, research facilities, and cultural events.

However, the greatest contribution of arthropods to human food supply is by pollination: a 2008 study examined the 100 crops that FAO lists as grown for food, and estimated pollination's economic value as €153 billion, or 9.5 per cent of the value of world agricultural production used for human food in 2005. [107] Besides pollinating, bees produce honey, which is the basis of a rapidly growing industry and international trade. [108]

The red dye cochineal, produced from a Central American species of insect, was economically important to the Aztecs and Mayans. [109] While the region was under Spanish control, it became Mexico's second most-lucrative export, [110] and is now regaining some of the ground it lost to synthetic competitors. [111] Shellac, a resin secreted by a species of insect native to southern Asia, was historically used in great quantities for many applications in which it has mostly been replaced by synthetic resins, but it is still used in woodworking and as a food additive. The blood of horseshoe crabs contains a clotting agent, Limulus Amebocyte Lysate, which is now used to test that antibiotics and kidney machines are free of dangerous bacteria, and to detect spinal meningitis and some cancers. [112] Forensic entomology uses evidence provided by arthropods to establish the time and sometimes the place of death of a human, and in some cases the cause. [113] Recently insects have also gained attention as potential sources of drugs and other medicinal substances. [114]

The relative simplicity of the arthropods' body plan, allowing them to move on a variety of surfaces both on land and in water, have made them useful as models for robotics. The redundancy provided by segments allows arthropods and biomimetic robots to move normally even with damaged or lost appendages. [115] [116]

Diseases transmitted by insects
Disease [117] Insect Cases per year Deaths per year
Malaria Anopheles mosquito 267 M 1 to 2 M
Dengue fever Aedes mosquito ? ?
Yellow fever Aedes mosquito 4,432 1,177
Filariasis Culex mosquito 250 M unknown

Although arthropods are the most numerous phylum on Earth, and thousands of arthropod species are venomous, they inflict relatively few serious bites and stings on humans. Far more serious are the effects on humans of diseases like malaria carried by blood-sucking insects. Other blood-sucking insects infect livestock with diseases that kill many animals and greatly reduce the usefulness of others. [117] Ticks can cause tick paralysis and several parasite-borne diseases in humans. [118] A few of the closely related mites also infest humans, causing intense itching, [119] and others cause allergic diseases, including hay fever, asthma, and eczema. [120]

Many species of arthropods, principally insects but also mites, are agricultural and forest pests. [121] [122] The mite Varroa destructor has become the largest single problem faced by beekeepers worldwide. [123] Efforts to control arthropod pests by large-scale use of pesticides have caused long-term effects on human health and on biodiversity. [124] Increasing arthropod resistance to pesticides has led to the development of integrated pest management using a wide range of measures including biological control. [121] Predatory mites may be useful in controlling some mite pests. [125] [126]

Even amongst arthropods usually thought of as obligate predators, floral food sources (nectar and to a lesser degree pollen) are often useful adjunct sources. [127] It was noticed in one study [128] that adult Adalia bipunctata (predator and common biocontrol of Ephestia kuehniella) could survive on flowers but never completed the life cycle, so a meta-analysis [127] was done to find such an overall trend in previously published data, if it existed. In some cases floral resources are outright necessary. [127] Overall, floral resources (and an imitation, i.e. sugar water) increase longevity and fecundity, meaning even predatory population numbers can depend on non-prey food abundance. [127] Thus biocontrol success may surprisingly depend on nearby flowers. [127]

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  2. ^ The fossil was originally named Eotarbus but was renamed when it was realized that a Carboniferous arachnid had already been named Eotarbus. [66]
  3. ^ For a mention of insect contamination in an international food quality standard, see sections 3.1.2 and 3.1.3 of Codex 152 of 1985 of the Codex Alimentarius[103]
  4. ^ For examples of quantified acceptable insect contamination levels in food see the last entry (on "Wheat Flour") and the definition of "Extraneous material" in Codex Alimentarius, [104] and the standards published by the FDA. [105]
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  • Definitions from Wiktionary
  • Media from Wikimedia Commons
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    at the Encyclopedia of Life chapter in United States Environmental Protection Agency and University of Florida/Institute of Food and Agricultural Sciences National Public Health Pesticide Applicator Training Manual Insect Life Forms

120 ms 5.4% Scribunto_LuaSandboxCallback::expandTemplate 120 ms 5.4% recursiveClone 100 ms 4.5% Scribunto_LuaSandboxCallback::gsub 80 ms 3.6% (for generator) 60 ms 2.7% Scribunto_LuaSandboxCallback::getAllExpandedArguments 60 ms 2.7% [others] 500 ms 22.3% Number of Wikibase entities loaded: 17/400 -->

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