Arthropoda: General Characters and Classification
Phylum Arthropoda (Gr., arthros, joint + podos, foot) is the largest phylum of Animal Kingdom including about 1,13.40,000 species in all habitats, which constitute about 83% of all the known species of animals.
General Characters:
1. Organ-system level of body organziation.
2. Body bilaterally symmetrical, triploblastic and metamerically segmented.
3. Appendages jointed, usually one pair to a somite, and with varied functions as jaws, gills, legs, etc.
4. Exoskeleton of dead chitinous cuticle that is shed at intervals, called ecdysis or moulting, for growth and development.
5. Body divisible into head, thorax and abdomen. Head and thorax often fused to form a cephalothorax.
6. True coelom reduced and largely replaced by a blood-filled haemocoel.
7. Muscles mostly striated, usually capable of rapid contraction.
8. Digestive system complete with mouth and anus. Mouth parts adapted for various modes of feeding.
9. Circulatory system open with a dorsal often many-chambered heart, arteries and blood sinuses or haemocoel.
10. Respiration by general body surface, gills, tracheae or book-lungs.
11. Excretory organs are green glands or Malpighian tubules.
12. Nervous system typically annelidan, with a dorsal brain connected with a nerve ring to a double ventral nerve cord.
13. Sensory organs comprises of eyes (simple and compound), chemo- and tactile receptors, and balancing and auditory organs.
14. Sexes usually separate (dioecious). Reproductive organs and ducts paired.Fertilization usually internal. Oviparous or ovoviparous.
15. Development direct or indirect with one to many larval stages. Parthenogenesis in some.
16. Cilia and absent except in Onychophora. 17. Parental care often well-marked.
X X
Classification:
Arthropoda is a much heterogeneous group including a variety of animals with divergent views concerning their phylogeny. Because of this reason, no definitive system of classifying this phylum exists.
The classification adopted here is a synthesis of several views so that such a large and diverse phylum may be conveniently grouped. In fact, arthropod classification is still in a state of flux, and may always remain so.
Seven subphyla are recognized in the following classification. Of these, only Trilobitomorpha, Chelicerata and Mandibulata are definitely arthropods. Trilobitomorpha includes a number of extinct classes only.
Onychophora, Tardigrada and Pentastomida show only doubtful or superficial relationships with other arthropods, so that some books treat them as independent minor phyla.
We have also described the type Peripatus under a separate phylum Onychophora, to accommodate this view. Pycnogonida is sometimes included as a class within the subphylum chelicerata.
The old class Myriapoda is retained here within the subphylum Mandibulata, otherwise, every order of Myriapoda is equivalent in status to other classes of Mandibulata.
(Gr., tri, three lobos, lobe + morphe, form)
1. Fossil trilobites. Mostly marine and bottom-dwellers. Cambrian to Permian.
2. Body 3-lobed, due to 2 longitudinal furrows.
3. Head distinct. Probably one pair of antennac.
4. Biramous appendages on all segments except the last one.
Examples: Triarthrus, Dalmanites.
1. Body divided into an anterior cephalothorax (prosoma) and (opisthosoma). a posterior abdomen 2.Prosomatic appendages 6 pairs. First pair of preoral chelicerac with claws, followed by postoral pedipalps and 4 pairs of walking legs.
3. Antennae and true jaws absent.
(Gr., meros, thigh+ stoma, mouth)
1. Marine with median simple and lateral compound eyes.
2. 5 to 6 pairs of abdominal appendages with book-gills.
3. Abdomen ending in a sharp telson or spine.
4. Excretion by coxal glands. No Malpighin tubules.
(Gr., xiphos, sword + aura, tail)
1. Cephalothorax with large extended, conver horseshoe-shaped carapace.
2. Abdomen unsegmented with a long terminal telson. Example: Limulus (horseshoe or king crab).
(Gr., eurys, broad + pteryx, wing).
1. Extinct marine, giant water scorpions.
2. Cephalothorax small. Carapace plain, not0extended. 12-segmented and narrowed
3. Abdomen behind.
Examples: Eurypterus, Pterygotus.
(Gr., arachne, spider)
1. Terrestrial or aquatic. Eyes simple. No compound eyes.
2. Cephalothorax (prosoma) with 2 chelicere, 2 pedipalps and 4 pairs of walking legs.
3. Abdomen generally without appendages 4. Respiration by tracheae, book-lungs or both.
5. Excretion by coxal glands and malpighian tubules.
6. Dioecious. Mostly oviparous courtship before mating.
(=Scorpiones)
1. Elongated, fair-sized true scorpions.
2 Small prosoma broadly joined to large opisthosoma, which is made of a broad
anterior 7-segmented mesosoma, and anarrow posterior 5-segmented metasoma.
3. Metasoma ending in a telsion and poison sting.
4. Two ventral comb-like sensory pectines on 2nd abdominal segment.
5. Respiration by 4 pairs of book lungs.
Examples:Buthus, Palamnaeus, Androctonus, Centruroides (= Centrurus).
(=Chelonethida)
1. Tiny false scorpions. Abdomen 11- segmented, without sting and telson.3. Chelicerae
2. jointed, with comb-like serrations. Examples: Chelifer, Microcreagris.
Order 3. Araneae
1. True spiders Prosoma and opisthosoma without visible segments and joined by anarrow pedicel.
2. Chelicerae 2-jointed. With a poison duct in terminal claw.
3. Pedipalps leg-like, used for transfer of sperms in male.
4. Opisthosoma with 3 pairs of spinnerets. No telson. Examples: Lycosa (wolf spider), Agelena (funnel-web spider), Latrodectus (black widow), Achacaranea (house spider), Argiope (writing spider).
Order 4. Solifugida (=Solifugae)
1. False spiders, sun spiders or wind spiders. 2. divided into a large anterior and small posterior part.
3. Opisthosoma of 10 or 11 segments. No spinnerets.
4. Chelicerae very large forming heavy pincers. No poison glands.
5. A flagellum on each chelicera of male, for sperm transfer.
Example: Galeodes.
Order 5. Palpigradi
1. Small-sized microwhip scorpions. Without eyes.
2. Prosomal carapace made of large anterior and small posterior portions.
3. Opisthosoma 11-segmented, ending in a large 15-jointed telson or flagellum.
Example: Koenenia.
X X
Order 6. Pedipelpi (= Uropygi)
1. Elongate whip scorpions with 1 pair of eyes.
2. Prosomal carapace entire.
3. Opisthosoma 12-segmented. Last segment with a long flagellum or telson.
4. Pedipalps large, heavy, with terminal pincer.
Examples: Mastigoproctus, Thelyphonus.
(= Phrynichida)
1. Flattenedscorpion-spiders or taillers whip scorpions.
2. Carapace undivided. Pedipalps large and rhaptorial.
3. Abdomen 12-segmented, without flagellum.
4. First pair of legs long, whip-like, sensory. Example: Charinus.
Order 8. Ricinulei (= Podogna)
1. Rare, small, tick-like, heavy-bodied arachnids.
2. Carapace with an anterior hood-like movable plate.
3. Opisthosoma 6-segmented. Narrow anteriorly with a posterior tubercle bearing anus.
4. Third pair of legs in male form copulatory organs.
Examples: Ricinoides, Cryptocellus.
Order 9. Opioliones (= Phalangida)
1. Spider-like Harvest-men, Harvest-spiders or daddy longlegs.
2. Body small, oval. Legs extremely long. slender.
3. Prosoma broadly joined to Opisthosoma. 4. Scent glands under carapace. Two eyes. Examples: Phalangium, Leiobunum.
Order 10. Acarina
1. Ticks and mites. Free-living or parasitic.
2. Body small, oval, unsegmented, with distinction between prosoma opisthosoma. 3. Largest arachnid order with 20,000 species.
Examples: Sarcoptes (Itchomite), Ixodes (Sheep tick), Dermacentor (Dog tick), Argas (Bird tick).
(L., mandibula, mandible+ata, group)
1. Body divisible into head, thorax and abdomen.
2. Head appendages are 1 or 2 pairs of antennae, 1 pair of jaws or mandibles and 1 or 2 pairs of maxillac.
3. Compound eyes common.
Class 1. Crustacea (Lcrusta, shell)
1. Head often joined with thorax to form cephalothorax.
2. Exoskeleton chitinous, hard, limy (calcareous).
3. Head 5-segmented, bearing 2 pairs of antennae, 1 pair of mandibles and 2 pairs of maxillac. Appendages typically biramous
4. Respiration by gills or body surface. 5. Excretion by antennal glands.
6. Sexes usually separate. Development with nauplius stage.
Subclass 1. Cephalocarida
1. Body made of a horseshoe-shaped head and 19 trunk segments, Only anterior 9 trunk segments bear appendages that appear triramous.
2. Antennae short. Eyes absent.
3. Hermaphrodite. Larva a metanauplius.
Example: Hutchinsoniella.
Subclass 2. Branchiopoda
1. Primitive, small-sized, mostly freshwater. 2. Trunk appendages leaf-like, serving for respiration (gills), locomotion and filter- feeding
3. Antennules and 2nd maxillae reduced or absent.
4. Abdomen ends in a pair of jointed or unjointed caudal styles or cercopods.
Order 1. Anostraca
1. Fairy shrimps with 19 or more trunk segments. Only anterior 11 to 19 segments bear appendages.
2. Carapace absent. Eyes stalked. Styles un- jointed.
Examples: Artemia, Eubranchipus.
Order 2. Notostraca
1. Tadpole shrimps with 25-45 trunk segments Anterior half with 35 to 71 pairs of appendages.
2. Carapace shield-like. Eyes sessile. Styles jointed.
Examples: Apus, Lepidumus.
Order 3. Diplostraca
1. Clam shrimps and water fleas with a bivalved carapace enclosing body with or without head.
2. Eyes fused, sessile. Styles unjointed. claw-like.
Examples: Daphnia, Cyzicus (- Estheria)
Subclass 3. Ostracoda
1. Minute mussel or seed shrimps with poorly segmented body entirely enclosed in a bivalved carapace.
2. Trunk appendages 2 pairs, leg-like.
3. Antennules and antennac large, used in swimming.
Order 1. Myodocopa
1. Carapace with antennal notches.
2. Antennae biramous, enlarged at base. Example: Cypridina.
Order 2. Podocopa
1. Carapace unnotched. Trunk appendages 2 pairs.
2. Antennae uniramous, clawed at tips. Examples: Cypris, Darwinula.
Order 3. Platycopa
1. Carapace unnotched. Trunk appendages 1 pair.
2. Antennae uniramous.
Example: Cytherella.
Order 4. Cladocopa
1. Carapace unnotched.
2. Antennae biramous.
Example: Polycope.
Subclass 4. Mystacocarida
1. Primitive. Body microscopic. Antennules and Antennae prominent.
2. A single median eye. No compound eyes. 3. Abdomen limbless. A pair of caudal styles Example: Derocheilocaris.
Subclass 5. Copepoda
1. Body small, made of head, thorax and abdomen.
2. No carapace. No compound eyes but a median eye.
3. Antennules long. Antennae smaller.
4. Abdomen limbless. Telson with two caudal styles.
Examples: Cyclops, Ergasilus, Caligus.
Subclass 6. Branchiura
1. Fish lice. Temporarily ectoparasites of skin and gill chambers of fishes and some amphibians.
2 Body dorso-ventrally flattened.
3. Shield-like carapace covers head and thorax.
4. A pair of sessile compound eyes. Mouth suctorial.
5. Antennules and antennae reduced.
6. First maxillae modified into suckers.
7. Abdomen unsegmented, bilobed. Caudal claws minute. Examples: Argulus, Dolops.
Subclass 7. Cirripedia
1. Barnacles, Adults sessile, attached orparasitic.
2. Carapace forms two folds of mantle surrounding body and covered externally by calcareous plates.
3. Thoracic limbs 6 pairs, biramous and cirriform.
4. Antennules become cement glands for attachment.
5. Antennae and compound eyes are lost in adult.
6. Abdomen rudimentary with caudal styles.
7. Nauplius larva passes through a cypris stage.
Order 1. Thoracica
1. Non-parasitic. With or without stalk, Hermaphrodite.
2 Mantle present with calcareous plates. 3. Thoracic appendages 6 pairs, cirriform. Examples: Lepas, Balanus.
Order 2. Acrothoracica
1. Sessile. Bore into molluse shells or corals. Unisexual.
2. Mantle reduced to a chitinous attachment disc.
3. Trunk appendges usually 4 pairs, cirriform.
Examples: Alcippe, Cryptophialus.
Order 3. Ascothoracica
1. Parasitic in echinoderms and corals.
2. Mantle bivalved or saccular.
3. Often appendages are lost but antennules remain present.
Examples: Synagoga, Dendrogaster.
Order 4. Apoda
Parasitic without mantle and appendages. Examples: Proteolepis.
Order 5. Rhizocephala
1. Adult parasitic, degenerate, sac-like.
2. Peduncle forms root-like absorptive branches ramifying throughout host's tissues.
Examples: Sacculina.
Subclass 8. Malacostraca
1. Body large-sized. Typically made of 19 segments.
2. Head and one or more thoracic segments form cephalothorax.
3. Carapace well-formed or vestigial or absent.
4. Paired compound eyes stalked or sessile. 5. Abdomen ends in a telson. No caudal styles. Important orders are as follows:
Order 1. Nebaliacea
1. Carapace bivalved with an adductor muscle.
2. Abdominal segment 7 instead of 6.
Example: Nebalia.
Order 2. Mysidacea
1. Body clongated. Uropods form fan tail.
2. Carapace thin, covering mostly of thorax.
Example: Mysis (opossum shrimp).
Order 3. Cumacea
1. Head and thorax greatly enlarged.
2. Carapace fused to 3-4 thoracic segments.
3. Abdomen narrows. Uropods slender.
Examples: Diastylis, Cumopis.
Order 4. Isopoda
1. Wood lice. Body dorso-ventrally flattened.
2. Head and 1 or 2 thoracic segments form cephalothorax.
3. Carapace absent. Gills and heart abdominal.
Examples Oniscus, Asellus, Limnoria.
Order 5. Amphipoda
1. Sand hoppers. Body laterally compressed.
2. Carapace absent. Gills thoracic. Eyes sessile, lateral.
Examples: Gammarus, Caprella, Cyamus.
Order 6. Stomatopoda
1. Mantis shrimps. Body flattened. Carapace small.
2. Abdomen large, broader than cephalothorax.
3. Second maxillipedes raptorial. Gills abdominal.
Example: Squilla.
Order 7. Decapoda
1. Shrimps, crayfishes, Lobsters, prawns, crabs,etc.
2. Carapace well-developed. Usually enclosing gill chambers on sides of cephalothorax.
3. First 3 pairs of thoracic limbs form maxillipedes.
4. Gills usually in 3 series present on thorax.
5. Statocyst present. Larva typically a zoaca.
Suborder (a) Natantia
1. Body laterally compressed. Rostrum prominent.
2. Ploopods well-developed Modified for swimming.
Examples: Prawns (Palaemon, Penaeus, Macrobrachium), Shrimp (Leucifer, Crangon).
Suborder (b) Reptantia
1. Body dorso-ventrally flattened. Rostrum short or absent.
2. Pleopods reduced. Not modified for swimming.
Examples: Lobsters (Palinurus, Scyllanus, Homarus), Crayfish (Astacus, cambarus), True crabs (Cancer, Carcinus), Hermit crab, (Eupagurus), coconut crab (Hippa), Spider crab (Inachus).
X X
(G., myrios, ten thousand + podos, foot)
1. Exclusively terrestrial, air-breathing mandi- bulate arthropods.
2. Body worm-like, made of head and elongated trunk with many similar leg-bearing segments.
3. Antennae 1 pair, jaws 3 pairs, legs more than 11 pairs.
4. Respiration by tracheae. Spiracles arranged segmentally. Excretion by 1 or 2 pairs of Malpighian tubules. 6. Sexes separate, Gonad single. Gonoducts paired.
(Gr., diplos, double + pous, foot)
1. Millipedes. Body clongate, subcylindrical and divisible into 5-segmented head, 4-segmented thorax and 11 to 100-segmented trunk
2. Legs 2 pairs on each trunk segment (Diplopoda).
3. Mandibles and maxillae 1 pair (Dignatha). Examples: Spirobolus, Julus.
(Gr., cheilos, lip + pous (foot)
1. Centipedes. Body dorso-ventrally flattened and divisible into head and 15 to 173 trunk segments.
2. Legs 1 pair on each trunk segment.
3. Mandibles 1 pair. Maxillae 2 pairs (Trignatha)
4. First pair of legs form poison claws. Examples: Scutigera, Lithobius, Scolopendra.
(Gr., pauros, small + pous, foot)
Minute grub-like body divisible into head and 11-12 trunk segments with 9-10 pairs oflegs. No eyes.
Example: Pauropus.
(Gr., Syn, together + phylon, tribe)
Body slender made of head and 15-22 trunksegments with 10-12 pairs of legs. No eyes.
Example: Scutigerella.
(L. insectus, cut or divided)
1. Body made of head (6 fused segments). thorax (3 segments) and abdomen (up to 11 segments).
2. Head with compound eyes (1 pair), antenna (1 pair) mandibles (1 pair) and maxillae (2 pairs).
3. Mouth parts modified for different feeding habits.
4. Thorax with 3 pairs of jointed legs and tor 2 pairs of wings which may be absent.
5. Respiration by tracheac. Spiracles lateral.
6. Excretion by Malpighian tubules.
7. Unisexual.Fertilization internal. Development usually with metamorphosis.
Subclass 1. Apterygota (Ametabola)
1. Primitively wingless insects.
2. Abdomen with 2 cerci and style-like appendages.
3 Little or no metamorphosis.
Order 1. Protura
1. No antennac, true eyes and metamorphosis.
2 Abdomen of 11 segments plus a telson.
Example: Acerentulus.
Order 2. Collembola
1. No eyes, tracheac, Malpighian tubules and metamorphosis. Mouth parts chewing or sucking.
2. Abdomen 6-segmented, with a springing organ.
Examples: Springtails. Achorutes, Sminthurus.
Order 3. Thysanura
1. Body covered by minute silvery scales.
2 Antennae long. Mouth parts chewing.
3. Abdomen 11-segmented. Cerci and telson long
Example: Lepisma (silver fish)
Subclass 2. Pterygota (Metabola)
1. Wings present. Secondarily lost in some.
2. No abdominal appendages except cerci.
3. Metamorphosis complete or incomplete.
Division (a). Exopterygota (Heterometabola)
1. Wings develop externally as buds.
2. Metamorphosis gradual. Young stages are nymphs.
Order 1. Orthoptera
1. Wings 2 pairs. Forewings straight and leathery. Hindwings membranous and folded at rest.
2. Mouth parts chewing. Prothorax large. Hindlegs jumping.
Examples:Periplaneta (Cockroach), Gryllus or Acheta (Cricket), Mantis (Praying mantis), Phyllium (Leaf insect), Carausius (Stick insect).Romalia and Poecilocercus,(Grasshoppers),Schistocerca (Locust).
Order 2. Isoptera
1. Wings 2 pairs.Held flat on back. Or wingless.
2. Mouth parts chewing. Social insects with many castes.
Examples: Termites or white ants.
Order 3. Dermaptera
1. Forewings small, leathery. Hindwings large, semicircular.
2. Mouth parts chewing Forcep-like cerci at the tip of abdomen for offense and defense.
Example: Forficula (Earwig).
Order 4. Ephemeroptera
1. Wings 2 pairs, membranous. Forewings longer and triangular. Hindwings smaller and rounded.
2. Adult mouth parts vestigial. Mandibulate in nymphs.
3. Abdomen carries long cerci and caucal filament.
Example: Ephemera (Mayfly).
Order 5. Odonata
1. Wings 2 pairs, membranous. Eyes very large.
2. Mouth parts chewing. Predaceous.
Examples: Dragon flies, Damsel flies.
Order 6. Plecoptera
1. Wings 2 pairs, membranous, longer than body.
2. Antennae long. Mouth parts chewing.
3. Naiads usually with tracheal gills.
Example: Isoperla (Stonefly).
Order 7. Psocoptera (= Corrodentia)
1. Wingless or forewings larger than hindwings.
2. Antennae long. Mouthparts chewing. Cerci absent.
Examples: Book lice (wingless), Bark lice (winged).
Order 8. Mallophaga
1. Wings absent. Body small. Head large. Mouth parts chewing. Eyes degenerate. Legs clasping,
2. Ectoparasitic on skin, hairs and feathers of mammals and birds.
Examples: Bird lice, Biting lice (on mammals).
Order 9. Anoplura (= Siphunculata)
1. No wings. Body broad, flat. Head small. Mouth parts piercing and sucking.
2. Claws clinging to hairs. Ectoparasitic on mammals. Example: Pediculus (Human louse).
Order 10. Thysanoptera
1. Wings 2 pairs, similar, fringed withhairs.
2. Mouth parts rasping and sucking.
Example: Thrips.
Order 11. Hemiptera
1. Wings 2 pairs or wingless. Forewings thickened at base, membranous at tip (hemelytra).
2. Mouth parts piercing-sucking, forming jointed beak.
Examples: Bedbug (Cimex), Giant water bug(Belostoma), Water scorpion (Ranatra).
Order 12. Homoptera
1. Wingless or 2 pairs of uniform membranous wings.
2. Mouth parts form a piercing and sucking beak.
Examples: Cicadas, Aphids, Scale insects.
Division (b). Endopterygota (Holometabola)
1. Wings develop internally in pupal case.
2. Metamorphosis complete with larval and pupal stages.
Order 1. Neuroptera
1. Wings large, membranous, many-veined. 2. Antennae long. Mouth parts chewing. Cerci absent.
3. Larvae carnivorous. Abdominal gills in aquatic larvae.
Examples: Crysopa (Lacewing), Myrmeleon (Antlion).
Order 2. Coleoptera
1. Forewings leathery (clytra). Hindwings membranous, folding.
2. Antennae variously modified. Mouth parts chewing.
Examples: Beetles.
Order 3. Mecoptera
1. Wings long, similar, narrow, membranous. 2. Mouth parts chewing, on a prolonged beak.
3. In male, tip of abdomen curved sting-like. 'Example: Panorpa (Scorpion fly).
Order 4. Trichoptera
1. Wings long, hairy, folded roof-like over abdomen.
2. Antennae long. Mouth parts rudimentary.
3. Larva puppets within a tube of foreign particles.
Example: Caddis flies.
Order 5. Lepidoptera
1. Wings membranous, covered with over- lapping scales.
2. Mouth parts sucking, coiled under head,
3. Larva a caterpillar with chewing mouth parts Examples: Butterflies filamentous), Moths (antennae feathery).
Order 6. Diptera
1. Wings 1 pair. Hindwings as knob-like halteres.
2. Mouth parts piercing-sucking or sponging.
3. Larva limbless, wormlike, called maggot. Examples: Musca (House fly), Culex (Mosquito), Drosophila (Fruit fly).
Order 7. Hymenoptera
1. Wings 2 pairs, similar, membranous. On each side hooked together during flight.
2. Mouth parts sucking or chewing. Ovipositor of female usually forms a piercing sting.
3. Highly specialized. Some social in behaviour Examples: Apis (Honey bee), Vespa, Ants.
Order 8. Siphonaptera
1. Small. Laterally flattened. Secondarily wingless.
2. Mouth parts piercing-sucking. Legs long, leaping.
3. Ectoparasites on birds and mammals.
Examples: Pulex and Xenopsylla (Fleas).
Minor Doubtful Arthropoda :
(G., onychos = claw + phoros, bearing)
1. Terrestrial,primitive,worm-like, unsegmented.
2. Single pairs of antennae, eyes and jaws. 3. Numerous stumpy, unjointed clawed legs.
Example: Peripatus
Subphylum V. Tardigrada
1. Minute, aquatic. Segmentation indistinct. No antennae.
2. Mouth retractile, with a pair of horny stylets .
3. Four pairs of stumpy, unjointed, clawed legs .
4. No respiratory, circulatory and excretory organs.
Example: Macrobiolus (Water bear).
(= Linguatulida)
1. Vermiform, unsegmented, parasitic worms. No antennae.
2. Two pairs of ventral retractile hooks near mouth.
3. No respiratory, circulatory and excretory organs. Example Linguatula (Tongue worm).
(= Pentopoda)
1. Small, marine, spider-like. Abdomen vestigial.
2. Mouth on a long proboscis. 4 simple eyes.
3. Appendages include chelicerae, pedipalps, ovigerous legs (1 pair) and long walking legs (4 to 12 pairs).
4. No respiratory and excretory systems. Examples: Pycnogonum, Nymphon spiders).
(Reference: modern textbook of zoology Invertebrates by R.L.Kotpal, Rastogi publications ,Meerut, New Delhi)
View, Comments and share.....
0 Comments