Classification of Insects | Arthropods | Zoology

The classification of insects into various orders is based on the presence or absence of wings and their venation, type of mouth parts, type of metamorphosis and characteristics of antennae and tarsi. The class Insecta has two subclasses, Apterygota, the primitive wingless insects, and Pterygota, insects with wings (although some of them may be secondarily apterous).

Initially, there were 29 orders of insects. Recently, a new insect order (with only 3 members) has been added in 2002 and the order has been named as Mantophasmatodea, because of superficial resemblance to the preying mantids and phasmids, the stick insects. Unlike phasmids, these insects lack an elongate mesothorax and unlike mantids, these lack raptorial forelegs.

Of these orders, only the more important ones are discussed below:

(1) Thysanura (thusanos, A Fringe; Oura, a Tail):

This order includes the spring-tails. The median tail has bristle-like processes and on each side there are long cerci. The body is covered with scales; antennae are long and mouthparts are meant for biting. The two common species are Lepisma saccharina Linnaeus, often seen behind calendars on the walls, and Thermobia domestica (Packard), the fire-brat, which is found in warmer places.

(2) Ephemeroptera (Ephemeros, Living a Day; Pteron, a Wing):

Mayflies are short-lived, have soft bodies and large eyes. The wings are membranous. The nymphs are aquatic, with tracheal gills, and form an important food for fish, particularly trout.

(3) Odonata (Odontos, a Tooth):

This order includes dragonflies and damselflies; these are large insects with elongated and brilliantly coloured bodies. Their eyes are large and mouth parts are strongly toothed. The wings are membranous with numerous cross veins. The nymphs are aquatic. The suborder Anisoptera includes dragonflies and Zygoptera, the damselflies.

(4) Plecoptera (Plekein, to Fold; Pteron, a Wing):

The stoneflies are soft-bodied insects and have long thread-like antennae. Their wings are membranous and mouthparts are of the biting type. The nymphs are aquatic and breathe through gills.

(5) Orthoptera (Orthos, Straight; Pteron, a Wing):

It includes grasshoppers, locusts, crickets, etc. The insects of this order have biting and chewing mouthparts and the fore wings are modified into tegmina that lie straight. The hind legs are modified for jumping. The females have well-developed ovipositors which are often used for digging holes in the soil where eggs are deposited.

Stridulatory and auditory apparatuses are well developed in many of the species. There are three main groups of this order and they are abundant in the tropical areas. Those with 4- segmented tarsi, long antennae and laterally compressed ovipositors are the long-horned grasshoppers, e.g., katydid (Tettigoniidae). Those with 3-segmented tarsi, long antennae and long straight ovipositors are the crickets (Gryllidae).

The mole-cricket (Gryllotalpidae) also belongs to this group, but in this case the ovipositors are vestigial and the fore legs are fossorial, modified for digging. They generally live a subterranean life and may do serious damage to the roots of crops. In both these groups, sound is produced by rubbing the wings and the auditory organs are in the fore tibia.

The third group is that of the short-horned grasshoppers and locusts (Acrididae), with 3-segmented tarsi, short antennae and short stout ovipositors. They produce sound by rubbing the hind femur against the fore wings. The auditory organs are located on the first abdominal segment. The desert locust, Schistocerca gregaria (Forskal), is the best-known example and it is the most notorious pest and scourage throughout the ages.

It has two phases. In the solitary phase, it lives like other grasshoppers in its natural habitat in Arabia and northern Africa. In the gregarious phase, it moves in swarms, many kilometres long from one country to another, leaving not a single blade of green vegetation behind. In the past, it has been the cause of famines and the change of kingdoms. There are about 20,000 species of insects belonging to this order.

(6) Phasmida (Phasma, an Apparition):

The stick-insects and leaf-insects are well known for their imitation in body form by resembling twigs or green leaves. They are usually large, elongated, cylindrical and often apterous. Their mouthparts are of the biting type, the legs are similar and ovipositors are concealed. They live mostly in the tropics.

(7) Dermaptera (Derma, Skin; Pteron, a Wing):

The earwigs have leathery fore wings. The hind wings are large, membranous and, when at rest, lie folded underneath the fore wings. Their mouthparts are for biting and there is one pair of forceps at the end of the body. They feed on humus and live in decaying wood. The females show maternal instinct. There are about 900 species in the temperate and tropical parts of the world.

(8) Embioptera (Embia, Genus; Pteron, a Wing):

These insects are commonly known as the web- spinners and they live in silken galleries made on the bark of trees. They are elongated and soft- bodied, the males having two pairs of smoky wings which are more or less alike. The mouthparts are for biting. The tarsi are 3-segmented and the first segment of the first pair is swollen and carries silk glands. The females are wingless. They generally live in warmer parts of the world and comprise about 140 species.

(9) Dictyoptera (Dictyon, A Network; Pteron, a Wing):

It includes the cockroaches and mantids. Their fore wings are thickened to form tegmina, and the tarsi are 5-segmented. The mouthparts are for biting; the ovipositors are concealed. Their eggs are contained in ootheca. The cockroaches (Blattidae) comprise about 3,500 species, mostly in the tropics.

They are nocturnal and feed on stuff left in the kitchen and pollute human food with their excrements and impart a foul smell which is not lost even on cooking. The preying mantids (Mantidae) are carnivorous and have raptorial fore legs with which they catch and hold their prey. They abound in warmer parts of the world.

(10) Isoptera (Isos, Equal; Pteron, a Wing):

The termites or white ants are common in the tropics. They are social insects and live in large colonies, each colony having a single queen. Their body is soft which easily gets desiccated and, hence, they live in earthern galleries or mounds. Their mouthparts are of the biting type and they have two pairs of similar elongated wings placed in sockets which are easily shed after the first flight.

Caste system among them is highly developed. The most common forms are workers and soldiers which are both sterile females and males. The sexual forms may be winged (macropterous) or short-winged (brachypterous) or may be wingless (apterous).

The whiteants (Termitidae) may be subterranean (Coptotermes), may live in dry wood (Mastotermes), may form mounds (Odontotermes), or they may make huge termitaria (Nasutitermes). The queen is the life of the colony and if she dies the colony may perish.

In certain species, however, the death of the queen is not necessarily the end of the colony, as brachypterous forms may start laying eggs to maintain the colony. Practically, all termites feed on the cellulose obtained from dead plants, wood, wooden structures, books, etc. They also attack crops, particularly sugarcane and wheat sown under unirrigated conditions.

(11) Psocoptera (Psocus, Genus; Pteron, a Wing):

The book lice and their allies are small soft-bodied winged or apterous insects. Their mouthparts are of the biting type and they have no cerci. About 1,000 species are known.

(12) Mallophaga (Mallos, Hair; Phagein, to Eat):

The bird-lice or the biting lice have the biting type of mouthparts. They are ectoparasites of birds and a few of them are also found on mammals. The thoracic segments are more or less free and the tarsi may be 1-or 2-segmented. They attach eggs to the feathers of their hosts and complete life-cycle without dropping off. Separated from the host, the lice cannot live for long. The chicken louse (shaft louse), Menopon gallinae (Linnaeus) (Menoponidae) or the chick ‘mite’, as it is called sometimes, is an important pest of poultry.

(13) Siphunculata (Siphunculus, a Little Tube):

The sucking lice are important ectoparasites of man and domestic animals. They are wingless insects having piercing- and-sucking type of mouthparts, with which they suck the blood of their host. Their thoracic segments are fused and they have 1-segmented tarsi, and a single claw with the help of which they cling to the hair of their host. The body louse of man, Pediculus humanus Linnaeus (Pediculidae) and the cattle louse, Haematopinus (Haematopinidae) are the common examples of this order.

(14) Hemiptera (Hemi, Half; Pteron, a Wing):

The plant-bugs and their allies have piercing and sucking type of mouthparts and their fore wings are generally corneous. Many of the forms are apterous and in some, there may be an incipient pupal instar. There are two suborders, Heteroptera and Homoptera. In the former, the basal half of the fore wing is of a much harder consistency than the rest of it.

The plant bugs are medium insects and the important crop pests include the rice bug, Leptocorisa acuta (Thunberg) (Coreidae), the green potato bug, Nezara viridula (Linnaeus) (Pentatomidae), the red cotton bug, Dysdercus koenigii (Fabricius) (Pyrrhocoridae), the lace bug, Urentius sentis Distant (Tingidae), the dusky cotton bug, Oxycarenus laetus Kirby (Lygaeidae). The common bed bug, Cimex lectularius Linnaeus (Cimicidae) although wingless, also belongs to this suborder.

In the second suborder, Homoptera, the fore wings, when present, are of a uniform consistency and it includes such crop pests as leafhoppers (Cicadellidae), aphids (Aphididae), pyrilla (Fulgoridae), citrus psylla (Psyllidae), the whiteflies (Aleyrodidae) and the scales (Diaspididae). Such commonly known insects as cicadas (Cicadidae) and the lac insect (Lacciferidae) also belong to this suborder which is of great importance in agriculture. This order is known to contain 59,000 species.

(15) Thysanoptera (Thusanos, a Fringe; Pteron, a Wing):

The thripses have very characteristic narrow wings, with long fringes. They are minute insects with 6-9 segmented antennae. The mouthparts are stylet-like and are used for rasping and sucking plant sap. The tarsi are short-each ending in a vesicle. In the life-history of these insects, there is an incipient pupal instar.

The thripses are commonly seen in flowers, but they may also feed on leaves. The well-known pest species are the grapevine thrips, Rhipiphorothrips cruentatus Hood (Heliothripidae) and onion thrips, Thrips tabaci Lindeman (Thripidae).

(16) Neuroptera (Neuron, a Nerve; Pteron, A Wing):

This order includes the heterogeneous group of alder flies, lace wings, ant lion, etc., which have two pairs of membranous wings, with many accessory branches and cross veins. They are small to large insects with soft bodies, and their larvae are campodeiform with biting or suctorial mouthparts, and are generally predacious. The larvae of lace wings destroy a large number of aphids, and those of the ant lions dig typical funnel-shaped pits in the sand and wait for stray ants to fall in and serve as their prey.

(17) Lepidoptera (Lepidos, a Scale; Pteron, a Wing):

The moths and butterflies whose bodies are covered over with scales belong to this order. They are small to very large insects in which the galeae are modified to form a spirally coiled sectorial proboscis used for sucking nectar from flowers and in some cases the fruit juice. Their larvae are active, mostly phytophagous, and some also feed on wax and cloth. Their pupae are object or partially free and are usually enclosed in cocoons.

It is very large order of 1,19,000 species and many of them are brilliantly coloured. The lepidopteran larva has a well-developed head, 3 thoracic and 10 abdominal segments. Abdominal feet or prolegs are present on segments 3-6 and 10. In some, the numbers of pairs are reduced and consequently, as they walk they form a loop of the body and are known as semiloopers or loopers.

Distinguishing Characteristics of Moths and Butterflies:

Moths:

i. Eggs – Generally flat and round

ii. Larvae – Generally covered with hairs

iii. Ocelli – Two ocelli present

iv. Type of antennae – Thread-like, comb like or feathery

v. Pupation – Pupae are mostly enclosed in silken cocoons

vi. Shape of body – Relatively large body

vii. Pattern of resting position of the wings – The wings lie in a slanting roof-like disposition on either side of the body-length

viii. Activity time – Usually night-fliers

Butterflies:

i. Eggs – Cigar shaped and cylindrical

ii. Larvae – Smooth and naked

iii. Ocelli – Absent

iv. Type of antennae – Club-shaped

v. Pupation – Pupae are naked

vi. Shape of body – Slender body

vii. Pattern of resting position of the wings – The wings are held over the back in the vertical position

viii. Activity time – Day-fliers

As pests of crops, this is the most important order and in a tropical country, such as India, several hundred species are known pests of economic plants. Out of these, over 50 are major pests. Some of the commonest pests are the cutworms, Agrotis spp. (Noctuidae), the sugarcane top borer, Scirpophaga (=Tryporyza) nivella (Fabricius) (Pyralidae), the maize borer, Chilo partellus (Swinhoe) (Pyralidae), the clothes moth, Tineola bisselliella (Hummel) (Tineidae), the red hairy caterpillar, Amsacta moorei Butler (Arctiidae), the cabbage butterfly, Pieris brassicae (Linnaeus) (Pieridae), the citrus caterpillar, Papilio demoleus Linnaeus (Papilionidae), the hawk moths (Sphingidae) and numerous others. The silk moth, Bombyx mori (Linnaeus) (Bombycidae), has a great economic importance as a producer of commercial silk.

(18) Diptera (Dis, Two; Pteron, a Wing):

The flies and its allies have two wings; the hind pair is modified to form halters used in balancing the body. Their mouthparts are modified into a proboscis used for piercing and sucking or sponging liquid foods. The typical larva is worm-like, legless, and has an indistinct head.

It has a terrestrial, aquatic or parasitic mode of life, the pupa is weakly object or exarate, enclosed in a puparium made of the last larval skin. There are three suborders- Nematocera, in which the larva has a well-developed head; Brachycera, in which the head is incomplete and retractile; and Cyclorrhapha, in which the head is vestigial.

The mosquito (Culicidae) belongs to the first suborder; horse flies (Tabanidae) and robber flies (Asilidae) belong to the second; the hover flies (Syrphidae), leafminers (Agromyzidae), blow flies (Calliphoridae) and the house fly (Muscidae) belong to the third suborder. This is a very large and important order comprising about 1,19,000 species.

(19) Siphonaptera (Siphon, a Tube; Apterous, Wingless):

The fleas which are included in this order have piercing-and-sucking mouthparts, with which they suck blood from the body of warm-blooded animals. The adults are ectoparasites with the body laterally compressed. The rat flea, Xenopsylla cheopis (Rothschild) (Pulicidae) frequently migrates to man and transmits the bacillus of bubonic plague.

(20) Hymenoptera (Hymen, a Membrane; Pteron, a Wing):

The ants, bees, wasps, sawflies, etc. have membranous wings, the hind pair being smaller than the fore wings. Their mouthparts are of the biting type and sometimes of the lapping or sucking type. The first segment of the abdomen is fused with thorax, often like a narrow petiole. The pupae are generally enclosed in cocoons.

Hymenoptera is divided into two suborders. Symphyta includes the wood-boring wasps. The female has ovipositors adapted for sawing and boring, as in the metallic-blue Sirex noctilio Fabricius (Siricidae), an important pest of timber in many parts of world. Apocrita includes the majority of Hymenoptera and the members are recognized from their constricted or petiolated abdomen.

The honey bee, Apis cerana Fabricius (Apidae), the red wasp, Vespa orientalis Linnaeus (Vespidae), the ants (Formicidae) and the parasitic wasps (Ichneumonidae and Chalcididae) are the common examples. The honey bees produce honey, which is a source of income of many apiculturists.

With the introduction of Apis mellifera Linnaeus, all four species of Apis are now found in India. The parasitic insects of this order are of great importance to agricultural entomologists interested in the biological control of insect pests. About 1,95,000 species of this order have been described so far.

(21) Coleoptera (Koleos, a Sheath; Pteron, a Wing):

This is the largest order of the animal kingdom and over 2,20,000 species have so far been described. The beetles are minute to large and have their fore wings modified to form elytra, which are hard and lie flat on the body, the membranous hind wings are folded underneath. Their prothorax is large and mouthparts are of the biting type. Metamorphosis is complete and the larvae are compodeiform or eruciform.

Most of the beetles are ground dwellers, some are aquatic and others are important pests of stored grains and other products. There are two main suborders, Adephaga and Polyphaga; the predacious tiger beetles (Cicindelidae) belong to the former. Most of the commonly known beetles belong to Polyphaga, which includes the ber beetle, Adoretus pallens Arrow (Scarabaeidae), the khapra beetle, Trogoderma granarium Everts (Dermestidae), the ladybird beetle, Coccinella septempunctata Linnaeus (Coccinellidae), wheat flour beetle, Tribolium castaneum (Herbst) (Tenebrionidae), the gram dhora, Callosobruchus chinensis (Linnaeus) (Bruchidae), the red pumpkin beetle, Raphidopalpa foveicollis (Lucas) (Chrysomelidae), the granary weevil, Sitophilus granarius (Linnaeus), and the rice weevil, Sitophilus oryzae (Linnaeus) (Curculionidae). Many of the pests of stored grains are now distributed throughout the world. This is the most diverse order containing 3,49,000 species.

(22) Strepsiptera (Stylopids):

Stylops sp. is the most important parasite of the nymphs of the sugarcane pyrilla. Its larvae are endoparasitic and the male adults are free living. In the adults, fore wings are modified into small clubs and are known as pseudo-halteres. The hind wings are large and fan- shaped. The females live in the host inside puparia. It is a small order of about 300 species.