Phylum Arthropoda: Definition, Characteristics, Classification and Examples

Phylum Arthropoda (Gr., arthros = jointed ; podos = foot) is the largest and most diverse group of animals in the animal kingdom, characterized by their segmented bodies, jointed appendages, and exoskeleton made of chitin. Members of this phylum exhibit bilateral symmetry, a well-developed nervous system, and an open circulatory system. Arthropods are highly adaptable and found in nearly all habitats, including terrestrial, aquatic, and aerial environments. This phylum includes insects (e.g., butterflies, beetles), arachnids (e.g., spiders, scorpions), crustaceans (e.g., crabs, lobsters), and myriapods (e.g., centipedes, millipedes). Arthropods play essential ecological roles, such as pollinators, decomposers, and as part of the food web.

General Characteristics of Phylum Arthropoda

* Phylum Arthropoda is the largest phylum of Animal Kingdom which constitute about 83% of all the known species of animals.

• Organ-system level of organization.
• They are triploblastic, bilaterally symmetrical, metamerically segmented animals.
• Exoskeleton of dead chitinous cuticle that is shed at intervals, called ecdysis or moulting, for growth and development.
• Body segments usually bear paired lateral and jointed appendages with varied functions as jaws, gills, legs, etc.
• Body is divisible into head, thorax and abdomen. Head and thorax often fused to form a cephalothorax.
• Body cavity is haemocoel. True coelom is reduced to the spaces of the genital and excretory organs and largely replaced by blood-filled haemocoel.
• Muscles mostly striated, usually capable of rapid contraction.
• Digestive tract complete with mouth and anus. Mouth parts adapted for various modes of feeding.
• Circulatory system open with a dorsal often many-chambered heart, arteries and blood sinuses or haemocoel but without capillaries.
• Respiration through general body surface, by gills in aquatic forms, trachea or book lungs in terrestrial forms.
• True nephridia are absent. Excretion by coelomoducts or malphigian tubules or green glands or coxal glands.
• Nervous system typically annelidan, with a dorsal brain.
• Sensory organs comprises of eyes (simple and compound), chemo- and tactile receptors, balancing and auditory organs.
• Cilia are entirely absent from all parts of the body except Onychophora.
• Sexes are generally separate and sexual dimorphism is often exhibited by several forms.
• Fertilization is internal. Development direct or indirect with one to many larval stages.
• Parental care often well-marked.

Classification of Phylum Arthropoda

Phylum Arthropoda is classified into five classes:

Class 1 🡪 Crustacea

Class 2 🡪 Myriapoda

Class 3 🡪 Insecta (Largest class in the animal kingdom)

Class 4 🡪 Arachnida

Class 5 🡪 Onychophora – *It is now considered as a separate phylum

CLASS 1. CRUSTACEA

1. Crustacea are mainly aquatic, mostly marine, few freshwater and few live in moist places on land, while few parasitic forms.
2. Body is covered externally by chitinous cuticle.
3. Body is bilaterally symmetrical, triploblastic and divisible into three regions head, thorax and abdomen.
4. Head consists of six fused anterior segments and bears a median eye, a pair of compound eyes and five pairs of appendages, i.e., a pair of antennules, a pair of antenna, a pair of mandibles and two pairs of maxillae.
5. Head is often fused with few or all the thoracic segments to form the cephalothorax.
6. Thorax and abdomen bear a pair of biramous appendages in each segment.
7. Respiration takes place either by the general surface of the body or by gills.
8. Body cavity is divided into compartments, most of which contain blood. True coelom is greatly reduced and is represented by the cavities of reproductive organs.
9. Blood vascular system comprises a dorsal contractile heart communicating by valvular ostia with an enclosing pericardial sinus.
10. Excretory organs are particularly modified coelomoducts which may be either maxillary glands or antennary (green) glands.
11. Nervous system consists of a brain united by oesophageal connectives with a ventral nerve cord.
12. Sexes are separate or united. Sexual dimorphism is common.
13. Development includes metamorphosis with free larval stage.

Subclass I. BRANCHIOPODA

1. Branchiopoda are mostly freshwater, free-living forms.
2. Body is composed of varying number of segments.
3. Carapace in the form of dorsal shield or bivalved shell is usually present.
4. Thoracic appendages are flat-lobed and leaf-like provided with bristles.
5. Abdomen is devoid of appendages but provided with a pair of caudal styles.
6. Antennules and maxillae are reduced or absent.
7. Parthenogenesis is very common.
8. Larva is nauplius.

Order 1. Anostraca

1. Carapace is not developed.
2. Body is elongated.
3. Eyes are stalked.
4. Antennae are uniramous.
5. Caudal styles are unjointed. Examples: 1. Branchipus, 2. Streptocephalus.

Order 2. Notostraca

1. Carapace is broad, shield-shaped.
2. Body is elongated.
3. Eyes are sessile.
4. Antennae are reduced.
5. Caudal styles are many jointed. Examples: 1. Apus, 2. Lepidurus.

Order 3. Conchostraca

1. Carapace is transparent bivalved shell enclosing the entire animal.
2. Eyes are sessile.
3. Antennae are large and biramous.
4. Caudal styles are unjointed curved claw-like. Examples: 1. Limneus, 2. Estheria.

Order 4. Cladocera

1. Carapace bivalved usually covers the body but not head.
2. Eyes are sessile and united together.
3. Antennae are biramous and used for swimming.
4. Caudal styles are unjointed claw-like.

Subclass II. OSTRACODA

1. Ostracoda are small crustaceans occurring both in freshwater and sea.
2. Body is laterally compressed, not distinctly segmented.
3. Carapace is bivalved enclose the head and trunk.
4. Two pairs of stout and cylindrical thoracic appendages.
5. Abdomen is rudimentary.
6. Liver and gonads extend into the valves of the carapace.
7. Antennules and antennae are used in swimming.
8. Mandibles are provided with palps.
9. Parthenogenesis is common.
10. Larva is nauplius. Example: Cypris.

Subclass III. MYSTACOCARIDA

1. Small crustaceans with an elongated body divisible into head, five segmented thorax and six segmented abdomen.
2. The last four thoracic segments and all the abdominal segments are free.
3. The head appendages are well developed.
4. There are no abdominal appendages except for the last abdominal segment which carries a pair of caudal styletes.
5. Only nauplius eye is present.
6. Sexes are separate. Example: Derocheilocaris.

Subclass IV. COPEPODA

1. Free or parasitic crustaceans found both in fresh and salt-water.
2. Body is small elongated and segmented.
3. Six pairs of thoracic appendages.
4. Abdomen without appendages but provided with a pair of caudal styles.
5. Carapace and compound eyes are absent.
6. Antennules and antennae are usually well developed.
7. Mandibles may be provided with a palp.
8. Genital apertures are situated on the seventh body segment.
9. Eggs are carried by the female.
10. Larva is nauplius. Example: Cyclops.

Subclass V. BRANCHIURA

1. Parasitic crustaceans living as ectoparasites on both freshwater and marine fishes.
2. Body is small and flattened.
3. Carapace in form of a lateral expansion of the head.
4. Mouth is suctorial.
5. Five pairs of thoracic appendages.
6. Compound eyes are present.
7. Genital aperture is situated on the fifth body segment. Example: Argulus.

Subclass VI. CIRRIPEDIA

1. Marine crustaceans commonly known as barnacles.
2. Adults are always fixed or may be parasitic.
3. Body is imperfectly segmented.
4. Carapace forms a pair of folds, the mantle, which completely encloses the animal.
5. Compound eyes are absent in adults.
6. Typically six pairs of biramous thoracic limbs.
7. Abdomen is rudimentary but is usually provided with a pair of caudal styles.
8. Hermaphrodite.

Order 1. Thoracica

1. Non-parasitic sedentary marine barnacles.
2. Body is surrounded by calcareous shell.
3. Six pairs of biramous thoracic limbs.
4. Alimentary canal present. Examples: 1. Lepas, 2. Balanus.

Order 2. Acrothoracica

1. Parasitic, boring in the shells of Mollusca.
2. Body is covered by large mantle.
3. Calcareous shell is absent.
4. Less than six pairs of thoracic appendages.
5. Alimentary canal present. Example: Alcippe.

Order 3. Ascothoracica

1. Parasitic in Anthozoa and Echinodermata.
2. Six pairs of thoracic appendages.
3. Alimentary canal present.
4. Mouth appendages are modified into piercing and sucking organs. Examples: 1. Petrarca, 2. Laura.

Order 4. Apoda

1. Parasitic.
2. Body without mantle and thoracic appendages.
3. Body maggot-like.
4. Anus absent.
5. Hermaphrodite. Example: Proteolepas.

Order 5. Rhizocephala

1. Parasitic on decapod crustaceans.
2. Body extremely degenerated.
3. Appendages and alimentary canal absent.
4. No segmentation.
5. Attached by a stalk with roots which penetrate into tissue of host. Example: Sacculina.

Subclass VII. MALACOSTRACA

1. Mostly large crustaceans found both in fresh and sea-water.
2. Body is always distinctly segmented.
3. Thorax comprises eight segments, while abdomen comprises six rarely seven segments.
4. Exoskeleton of head united with few or more thoracic segments to form cephalothoracic carapace.
5. Compound eyes are usually stalked.
6. Abdomen is devoid of caudal styles.
7. Development through zoea stage. Nauplius stage rarely occurs.

Order 1. Nebaliacea

1. Primitive marine crustaceans.
2. Large bivalved carapace with an adductor muscle.
3. Thorax with eight pairs of leaf-like gills.
4. Abdomen contains seven segments and a telson.
5. Stalked eyes.
6. Telson with a pair of caudal style. Example: Nebalia.

Order 2. Mysidacea

1. Marine crustaceans.
2. Carapace covers almost entire thorax.
3. Eyes stalked.
4. Thoracic appendages are all biramous.
5. Uropods form broad fan-like tail fin. Example: Mysis.

Order 3. Isopoda

1. Marine, freshwater, terrestrial or parasitic crustaceans.
2. Body dorsoventrally flattened.
3. Carapace absent.
4. Eyes sessile.
5. Abdomen is usually short. Examples: 1. Oniscus, 2. Bopyrus.

Order 4. Amphipoda

1. Mostly marine crustaceans.
2. Body usually laterally compressed.
3. No carapace.
4. Eyes sessile.
5. Second and third pair of thoracic appendages are nearly always modified as prehensile organs. Examples: 1. Caprella, 2. Gammarus.

Order 5. Stomatopoda

1. Marine forms.
2. Body elongated with broad abdomen.
3. Carapace small covering three thoracic segments.
4. Stalked eyes.
5. First five pairs of abdominal appendages are provided with gills.
6. Heart is elongated. Example: Squilla.

Order 6. Decapoda

1. Mostly marine forms.
2. Carapace covers the entire thorax.
3. Thoracic appendages are modified as three pairs of maxillipedes and five pairs of walking legs.
4. Statocyst present.

Suborder 1. Macrura

1. Abdomen well developed, elongated and extended.
2. Antennules and antennae are both large.
3. Eyes are not enclosed in orbits. Examples: 1. Palaemon, 2. Astacus.

Suborder 2. Anomura

1. Abdomen more or less reduced and fixed. Examples: 1. Hippa, 2. Eupagurus.

Suborder 3. Brachyura

1. Abdomen is greatly reduced, hard and folded beneath the body. Examples: 1. Cancer, 2. Carcinus.

CLASS 2. MYRIAPODA

1. Terrestrial and air-breathing arthropods.
2. Body is usually elongated with numerous segments each bearing one or two pairs of legs.
3. Head is distinct bearing many jointed antennae, a pair of eyes and two or three pairs of jaws.
4. Alimentary canal is simple, midgut without special digestive glands.
5. Respiratory organs are the tracheae with segmentally arranged spiracles.
6. Excretory organs are one or two pairs of Malpighian tubules associated with hindgut.
7. Sexes are separate.

Subclass I. PROGONEATA

1. Genital apertures are situated near the anterior end of the body.

Order 1. Pauropoda

1. Body with eleven trunk segments and anal pygidium.
2. Nine pairs of legs.
3. Head bears antennae and two pairs of jaws.
4. Tracheae and circulatory system absent. Example: Pauropus.

Order 2. Diplopoda

1. Body is usually cylindrical.
2. Head bears antennae, one pair of mandibles and one pair of maxillae.
3. Poison jaws are absent.
4. Trunk bears numerous segments, each of which bears two pairs of legs.
5. Genital opening on the third segment behind the head.
6. Herbivorous. Example: Julus.

Order 3. Symphyla

1. Head with antennae; one pair of mandibles and two pairs of maxillae.
2. Trunk with twelve leg bearing segments.
3. Single pair of branching tracheae. Examples: 1. Scolopendrella, 2. Scutigerella.

Subclass II. OPISTHOGONEATA

1. Genital apertures are situated at the posterior extremity of the body.

Order 1. Chilopoda

1. Body is usually dorsoventrally flattened.
2. Head bears many jointed antennae, a pair of mandibles and two pairs of maxillae.
3. Numerous trunk segments, each bearing a single pair of legs.
4. Poison jaws are present.
5. Genital opening at the hind end of the body.
6. Carnivorous. Examples: 1. Scolopendra, 2. Scutigera.

CLASS 3. INSECTA

1. Insects are air-breathing mostly terrestrial and rarely aquatic arthropods.
2. Body is divided into three distinct regions, head, thorax and abdomen.
3. Head consists of six fused segments and bears a pair of compound eyes, a pair of antennae and mouth parts adapted for chewing, biting, piercing, sucking, siphoning or sponging type.
4. Thorax comprises three free segments, each bearing a pair of legs and two pairs of wings borne on the second and third segments.
5. Abdomen comprises 7-11 segments and devoid of appendages.
6. Liver is absent but salivary glands are usually present.
7. Heart is elongated, tubular and is divided into eight chambers situated in the abdomen.
8. Respiration by branched tracheae.
9. Excretion by Malpighian tubules.
10. Sexes are separate.
11. Development is sometimes direct, more usually complicated by metamorphosis.

Subclass I. APTERYGOTA

1. Primitive wingless insects.
2. Metamorphosis is little or absent.

Order 1. Protura

1. Minute soft bodied insects.
2. Antennae and compound eyes are absent.
3. Mouth parts are biting.
4. Abdomen twelve segmented, first three segments bear a pair of one or two segmented appendages.
5. Tracheae absent.
6. Metamorphosis little. Example: Acerentomon.

Order 2. Thysanura

1. Small primitive insects.
2. Body is elongated, flattened and naked or scaly.
3. Mouth parts biting.
4. Antennae long and well developed. Compound eyes.
5. Abdomen eleven segmented, some of which may bear small appendages.
6. A pair of cerci many segmented and a median caudal filament long.
7. Primitive metamorphosis. Example: Lepisma.

Order 3. Aptera

1. Small, white or pale blind insects.
2. Body flattened.
3. Mouth parts biting.
4. Antennae long many segmented.
5. Caudal filament absent.
6. Metamorphosis absent. Example: Campodea.

Order 4. Collembola

1. Minute insects.
2. Body naked or covered with hairs of scales.
3. Antennae four to six segmented.
4. Eyes absent.
5. Mouth parts biting.
6. Abdomen six segmented and usually with a ventral tube, tenaculum and spring. 7

Subclass II. PTERYGOTA

1. Wings are usually present.
2. Abdomen devoid of appendages except genitalia and cerci.
3. Metamorphosis simple or complex.

DIVISION I. EXOPTERYGOTA

• Metamorphosis primitive or simple.
• Young stages are nymphs.
• Wings develop externally.

Order 5. Orthoptera

1. Medium-sized to large insects.
2. Usually with a pair of narrow leathery forewings or tegmina and a pair of well developed membranous hind wings.
3. Mouth parts strong biting and chewing.
4. Compound eyes and usually two or three ocelli.
5. Short or long, simple or segmented cerci.
6. Simple metamorphosis. Examples: Locusts, Grasshopper, Crickets, etc.

Order 6. Grylloblatodea

1. Small thysanuriform insects.
2. Wings absent.
3. Mouth parts biting type.
4. Antennae filiform, many segmented.
5. Compound eyes are small, ocelli absent.
6. Simple metamorphosis. Example: Grylloblatta.

Order 7. Blattaria

1. Medium to large-sized insects.
2. Wings present or absent.
3. Body wide and depressed with large pronotum projecting over the head.
4. Antennae long many segmented and chewing type.
5. Mouth part biting and filiform.
6. Cerci prominent and segmented.
7. Metamorphosis simple. Examples: 1. Periplaneta, 2. Blatta.

Order 8. Phasmida

1. Large leaf or stick-like insects.
2. Antennae usually long, filiform and many segmented.
3. Compound eyes are small. Ocelli two or three or absent.
4. Mouth parts biting type.
5. Wings present or absent.
6. Cerci small and segmented. Example: Phyllium.

Order 9. Mantodea

1. Medium to large insects.
2. Head small, triangular and bears large compound eyes with three or none ocelli.
3. Mouth parts biting type.
4. Prothorax greatly elongated.
5. Fore-legs modified for capturing and holding the prey.
6. Wings large, small or absent.
7. Metamorphosis is simple. Example: Mantis (Praying mantis).

Order 10. Dermaptera

1. Small to medium-sized insects.
2. Body is elongated with smooth or tough or chitinous covering.
3. Mouth parts biting type.
4. Abdomen is provided with forcep-like cerci at the posterior end.
5. Simple metamorphosis. Example: Forficula.

Order 11. Diploglossata

1. Very small insects.
2. Body depressed covered with short hairs.
3. Mouth parts biting type.
4. Eyes and wings absent.
5. A pair of unsegmented cerci present.
6. Simple metamorphosis. Example: Hemimerus.

Order 12. Plecoptera

1. Medium to large-sized, elongated, somewhat flattened and soft-bodied insects.
2. Head broad and bears a pair of long filiform 25 to 100 segmented antennae.
3. Compound eyes are moderate in size with two or three or no ocelli.
4. Mouth parts biting type.
5. Two pairs of equal many veined wings, hind pair with large anal area.
6. Abdomen eleven segmented, last segment reduced and bears long filiform multisegmented cerci.
7. Simple or hemimetabolous metamorphosis.
8. Larvae are known as naiads which are aquatic. Examples: Stone flies, Salmon flies.

Order 13. Isoptera

1. Social and polymorphic insects living in colonies under a caste system.
2. Small to medium-sized mostly soft-bodied and pale coloured insects.
3. Head small to very large bears moniliform short or long many segmented antennae.
4. Mouth parts biting and chewing type.
5. Compound eyes vestigial with two or no ocelli.
6. Wings when present, two pairs, similar in size, shape and venation.
7. Cerci short and simple or 2-8 segmented.
8. Metamorphosis simple.
9. A colony contains both winged and wingless sexual individuals and wingless sterile workers and soldiers. Examples: Termites, White ants.

Order 14. Zoraptera

1. Minute winged or wingless insects.
2. Antennae moniliform, nine segmented.
3. Compound eyes and ocelli are present in winged forms, absent in wingless forms.
4. Mouth parts biting type.
5. Cerci short and unsegmented. Example: Zorotypus.

Order 15. Embioptera

1. Small slender insects with somewhat flattened body.
2. Head large bears filiform antennae.
3. Compound eyes small in female, often large in male. Ocelli absent.
4. Mouth parts biting chewing type.
5. Thorax nearly as long as abdomen.
6. First tarsal segment of fore-legs enlarged to contain glands and spinnerets.
7. Wings when present two pairs, membranous, similar in size and venation.
8. Metamorphosis simple or hemimetabolous. Example: Oligotoma.

Order 16. Corrodentia

1. Small compact winged or wingless insects.
2. Head large and free bears short or long filiform antennae.
3. Compound eyes are usually large or without ocelli.
4. Mouth parts modified biting type.
5. Cerci absent.
6. Metamorphosis simple. Examples: Book lice, Bark lice, Dust lice.

Order 17. Mallophaga

1. Small wingless ectoparasitic insects.
2. Body is broad or elongated, flattened dorsally, tough and well chitinised.
3. Head hypognathus without setiform or clavate antennae.
4. Compound eyes are reduced, ocelli absent.
5. Mouth parts biting type.
6. Legs short with stout claws specialized for clasping hairs and feathers.
7. Cerci absent. Example: Bird lice.

Order 18. Anoplura

1. Minute to small-sized, permanently ectoparasitic on mammals.
2. Body is flattened, elongated without wings.
3. Head small with short 3-5 segmented, setiform antennae.
4. Compound eyes reduced or absent, ocelli absent.
5. Mouth parts piercing and sucking type.
6. Legs short with single claw specialized for grasping hairs.
7. Cerci absent.
8. Metamorphosis simple. Example: Pediculus (Human lice).

Order 19. Ephemerida

1. Small to medium-sized, soft-bodied delicate slender insects.
2. Antennae short, setiform.
3. Mouth parts vestigial.
4. Well developed compound eyes and three ocelli present.
5. One or two pairs of fragile many-veined wings.
6. Cerci are very long, filiform and multisegmented with a similar median caudal filament.
7. Metamorphosis simple or hemimetabolous.
8. Naiad (nymphs) are aquatic with abdominal tracheal gills. Example: Ephemera (May flies).

Order 20. Odonata

1. Medium-sized to large, slender swift flying predaceous insects.
2. Head mobile, bears large compound eyes and ocelli.
3. Mouth parts biting type.
4. Two pairs of similar long, narrow, net-veined wings.
5. Naiads (nymphs) are aquatic with or without exterior gills.
6. Metamorphosis simple or hemimetabolous. Examples: Dragon flies, Demsel flies.

Order 21. Thysanoptera

1. Minute, slender terrestrial insects.
2. Body somewhat compressed dorsally or almost cylindrical.
3. Antennae short six to nine segmented.
4. Compound eyes with three ocelli present.
5. Mouth parts modified for piercing, chafing and sucking.
6. Wings when present two pairs of long and narrow with few veins.
7. Abdomen 10-11 segmented with an ovipositor.
8. Simple metamorphosis. Example: Thrips.

Order 22. Hemiptera

1. Minute to large, oval or elongated and dorsoventrally flattened terrestrial or aquatic insects.
2. Antennae 2 to 10 or rarely 25 segmented.
3. Eyes large with or without ocelli.
4. Mouth parts piercing and sucking type.
5. Wings when present two pairs, fore-pair usually thickened basally and membranous apically in Heteroptera and wholly membranous in Homoptera.
6. Cerci absent.
7. Anal respiratory filaments present in some aquatic forms and anal filaments in male coccids.
8. Metamorphosis simple.
9. Mostly phytophagous some are predaceous. Examples: Belostoma, Aphids, Cicadas, Scale insects.

DIVISION II. ENDOPTERYGOTA

• Metamorphosis is complex (complete).
• Young stages are known as larvae.
• Wings develop internally.

Order 23. Megaloptera

1. Medium to large-sized insects.
2. Head prognathus bears many segmented slender antennae.
3. Mouth parts biting type.
4. Wings two pairs, similar in shape, size and venation.
5. Cerci absent.
6. Metamorphosis complex including aquatic larvae. Examples: 1. Sialis, 2. Corydalus.

Order 24. Neuroptera

1. Minute to medium-sized, mostly terrestrial insects.
2. Head hypognathus, bears large and widely separated compound eyes.
3. Antennae usually filiform.
4. Mouth parts biting type.
5. Wings two pairs, similar in shape, size and venation.
6. Cerci absent.
7. Complex metamorphosis.
8. Carnivorous. Example: Chrysopa.

Order 25. Raphidiodea

1. Small to medium-sized, elongated, fragile, terrestrial insects.
2. Antennae setiform, many segmented.
3. Compound eyes prominent with three or none ocelli.
4. Mouth parts biting type.
5. Two pairs of similar wings.
6. Female with long slender ovipositor.
7. Metamorphosis complex. Examples: Snake flies, Serpent flies.

Order 26. Mecoptera

1. Small to medium-sized, slender, predaceous insects.
2. Head usually elongated and vertical.
3. Antennae long, filiform and many segmented.
4. Compound eyes are large and widely separated.
5. Ocelli three or none.
6. Mouth parts biting type.
7. Wings two pairs, similar in shape, size and venation.
8. Cerci short, simple or two segmented.
9. Metamorphosis complex. Example: Panorpa (Scorpion flies).

Order 27. Trichoptera

1. Small to medium-sized, diurnal and nocturnal moth-like insects.
2. Antennae long, filiform and many segmented.
3. Compound eyes are well developed with three or none ocelli.
4. Mouth parts biting type.
5. Wings two pairs, covered with hairs and scales.
6. Cerci one or two segmented.
7. Metamorphosis complex or holometabolic.
8. Larva aquatic. Example: Philopotamus (Caddis fly).

Order 28. Lepidoptera

1. Medium to large-sized flying terrestrial insects.
2. Body and wings are completely covered by flat overlapping scales and hairs.
3. Antennae variable often clavate or serrate, hooked or knobbed or plumose.
4. Compound eyes large with two or no ocelli.
5. Mouth parts siphoning type with long coiled proboscis.
6. Wings two pairs, fore pair often largest.
7. Metamorphosis complex.
8. Larvae caterpillars with three pairs of thoracic legs and two to four pairs of abdominal prolegs. Examples: Butterflies, Moths.

Order 29. Coleoptera

1. Minute to large insects with leathery integument.
2. Antennae variable in shape and size, usually 11 segmented.
3. Eyes conspicuous but ocelli absent.
4. Mouth parts biting and chewing type.
5. Wings two pairs, fore-wings or elytra similar to body texture and useless in flight.
6. Abdomen usually 10 segmented.
7. Cerci absent.
8. Metamorphosis complex. Examples: Beetles, Weevils.

Order 30. Strepsiptera

1. Minute free-living and endoparasitic dimorphic insects.
2. Antennae short flabellate.
3. Eyes conspicuous but ocelli absent.
4. Mouth parts atrophied biting type.
5. Fore-wings small, while hind-wings large fan-shaped.
6. Metamorphosis complex and hypermetamorphosis. Example: Stylops.

Order 31. Hymenoptera

1. Minute to large-sized, social or parasitic insects.
2. Antennae 12 segmented in male and 13 segmented in female.
3. Ocelli generally present.
4. Mouth parts specialized for biting, chewing, lapping and sucking.
5. Wings two pairs, membranous and narrow.
6. Abdomen always with basal pedicel.
7. Female with conspicuous ovipositor modified into saw, drill or sting.
8. Metamorphosis complex. Examples: Wasps, Bees, Ants.

Order 32. Diptera

1. Minute to medium-sized, diurnal, nocturnal, terrestrial and aquatic insects.
2. Body slender with three ocelli.
3. Antennae variable, simple or with arista.
4. Eyes large and separate with three ocelli.
5. Mouth parts modified for sucking, lapping, piercing and sponging.
6. Fore-wings developed for flight, while hind-wings reduced to halteres.
7. Metamorphosis complex.
8. Larvae limbless and vermiform known as maggots. Examples: Houseflies, Mosquitoes, Midges.

Order 33. Siphonaptera

1. Minute to small-sized insects ectoparasitic on birds and mammals.
2. Body laterally compressed.
3. Head small with or without eyes and ocelli.
4. Antennae short and sucking type.
5. Mouth parts piercing and sucking type.
6. Cerci absent.
7. Metamorphosis complex. Example: Fleas.

CLASS 4. ARACHNIDA

1. Arachnida are air-breathing, mostly terrestrial arthropods.
2. Body is usually distinguishable in two regions, cephalothorax or prosoma and abdomen or opisthosoma.
3. Cephalothorax bears sessile usually simple eyes, two pairs of jointed appendages, the chelicere and pedipalpi and four pairs of legs.
4. Antennae and true jaws are absent.
5. Abdomen is usually without appendages but sometimes ends in a telson.
6. Respiratory organs are varied. In terrestrial forms are tracheae and book-lungs and in aquatic forms book-gills.
7. Heart is dorsal and tubular.
8. Excretory organs are Malpighian tubules or coxal glands or both.
9. Sexes are separate but sexual dimorphism is not conspicuous.
10. Development mostly direct.

Order 1. Eurypterida

1. Extinct (Palaeozoic) forms of large size.
2. Small cephalothorax, followed by twelve free segments and a terminal elongated, narrow telson.
3. Cephalothorax bears six pairs of appendages. Example: Eurypterus.

Order 2. Xiphosura

1. Marine arachnids burrowing in the sand.
2. Body consists of an anterior prosoma and a posterior opisthosoma.
3. Prosoma is convex covered by a broad carapace and bears six pairs of appendages.
4. Opisthosoma is hexagonal consisting of six mesosomatic segments and vestigial unsegmented metasoma with a long narrow telson.
5. Abdomen bears operculum.
6. Respiration by lamelliform gills or book-gills attached to the abdominal appendages. Example: Limulus (King-crab).

Order 3. Scorpionidea

1. Terrestrial arachnids found under stones in tropical and subtropical regions.
2. Body consists of prosoma, mesosoma and metasoma.
3. Prosoma bears a pair of chelicerae, a pair of pedipalpi and four pairs of walking legs.
4. Mesosoma is narrow consisting of seven segments.
5. Metasoma is narrow consisting of five segments with a poison sting.
6. A pair of comb-like pectines occur on the second segment of the mesosoma.
7. Respiration by four pairs of book-lungs. Examples: 1. Palamnaeus, 2. Buthus.

Order 4. Pedipalpida

1. Body consists of unsegmented prosoma and an opisthosoma.
2. Prosoma bears six pairs of appendages.
3. Opisthosoma consists of eleven to twelve segments.
4. Chelicerae are simple and non-chelate or clawed.
5. Pedipalpi are large and either chelate or clawed.
6. Respiratory organs are two pairs of book-lungs. Example: Phrynichus.

Order 5. Araneidea

1. Found under stones, bark and leaves, etc.
2. Body consists of prosoma and opisthosoma.
3. Prosoma is unsegmented covered by carapace and bears six pairs of appendages.
4. Opisthosoma is usually unsegmented attached to prosoma by a narrow pedicle.
5. Chelicerae are subchelate with poison glands.
6. Pedipalpi are simple and six jointed.
7. Eight eyes are arranged dorsally on the head in two rows.
8. Respiration either by book-lungs with tracheae. Examples: 1. Aranea, 2. Argiope.

Order 6. Palpigradi

1. Small arachnids.
2. Last two prosomal segments are free.
3. Abdomen consists of ten segments and is jointed to the prosoma by a pedicle.
4. Telson with a long jointed flagellum.
5. Chelicerae are chelate and pedipalpi leg-like.
6. Respiration by three pairs of book-lungs. Example: Koenenia.

Order 7. Solifugae

1. Body consists of prosoma and opisthosoma.
2. Prosoma consists of three fused and last three free segments.
3. Opisthosoma composed of ten segments is not marked off from prosoma.
4. Chelicerae are large and chelate.
5. Pedipalpi are elongated and leg-like.
6. Respiration by tracheae.
7. Poison glands are absent. Example: Galeodes.

Order 8. Pseudoscorpionidea

1. Minute arachnids having the appearance of scorpions, found under the bark of trees.
2. Body consists of a broad prosoma and an opisthosoma of 10-12 segments.
3. Chelicerae and pedipalpi are similar to those of scorpions.
4. Telson absent.
5. A pair of spinning glands present.
6. Respiration by tracheae. Example: Chelifer.

Order 9. Ricinulei

1. Body consists of prosoma and opisthosoma.
2. Prosoma with a movable anterior cucullus.
3. Opisthosoma composed of nine segments and is joined to prosoma by a pedicel.
4. Chelicerae and pedipalpi are chelate.
5. Respiration by tracheae. Example: Cryptocellus.

Order 10. Phalangida

1. Spider-like in appearance.
2. Pedicel absent.
3. Prosoma unsegmented.
4. Opisthosoma composed of ten segments.
5. Chelicerae are chelate and pedipalpi leg-like.
6. Spinning glands absent.
7. Respiration by tracheae. Example: Phalangium.

Order 11. Acarina

1. Body unsegmented in which prosoma is fused with opisthosoma.
2. Chelicerae and pedipalpi are usually small and associated with the mouth-parts which are adapted for biting or piercing.
3. Respiration by tracheae or by skin. Examples: 1. Ixodes (Tick), 2. Chorioptes (Mite).