Insects are the largest group of animals, which constitute 75% of all living animals. There are more than one million living species in the world and many more yet to be discovered. They were the first flying creatures and still the only invertebrates that can fly. They are the most successful animals inhabiting every conceivable ecological condition.
Although only 2% of the insect species are obnoxious to man, they are enough to cause heavy damage to crop, livestock and man himself. That makes them very important for human attention. Pest species are difficult to control and in spite of about a century of constant efforts, cockroaches, flies and mosquitoes are as abundant as ever. Insect bodies are extremely tough due to exoskeleton and they possess enormous muscle power. They are endowed with the capacity to survive in the most inhospitable situations.
Insect body is divided into head, thorax and abdomen. They have 3 pairs of legs and hence sometimes called Hexapoda. One pair of antennae functions as sensory organs. One pair of compound eyes and one to three pairs of ocelli (sometimes absent) may be present.
Mouthparts are variously adapted depending on the mode of feeding. Breathing is by tracheal system. Circulatory system is open type. They excrete uric acid through malpighian tubules. Nervous system includes a circumpharyngeal ring and a ventral nerve cord. Sense organs are very well developed. Reproductive capacity is enormous and larval stages grow by molting (ecdysis). In majority of them larval stage is different from the adult and there is a distinct metamorphosis.
Class Insecta is divided into two subclasses, namely, Apterygota and Pterygota.
Subclass Apterygota (=Ametabola)
Primitively wingless insects, without metamorphosis. Head is prognathous. Abdominal appendages may be present. Ectognathous mouthparts, with mandibles attached at one point. There is no metamorphosis as nymphs are similar to adults. The subclass has 4 orders.
1.OrderProtura (Telson tails)
2. Order Diplura (Campodeids and Japygids)
3. Order Collembola (Spring tails and Snow fleas)
4. OrderThysanura (Silver fish)
Subclass Pterygota (=Metabola)
Winged or secondarily wingless insects, with prognathous or hypognathous head. Mouthparts endognathous. Metamorphosis either incomplete without a pupal stage or complete with a pupal stage. This subclass is divided into two superorders or divisions, namely, Exopterygota and Endopterygota.
Division Exopterygota (=Hemimetabola)
Wings develop externally during the larval growth and there is no pupal stage. Nymphs possess compound eyes and have similar habits and habitats as the adults. There are 16 orders in this group.
1. Order Ephemeroptera (May flies)
2. Order Odonata (Dragon flies and Damsel flies)
3. Order Orthoptera (Grasshoppers, Locusts, Crickets, Mole crickets)
4. Order Dictyoptera (Cockroaches and Mantids)
5. Order Phasmida (Leaf insects and Stick insects)
6. Order Plecoptera (Stone flies and Salmon flies)
7. Order Isoptera (Termites)
8. Order Zoraptera (Zorotypus)
9. Order Embioptera (Web-spinner)
10. Order Dermaptera (Earwigs)
11. Order Psocoptera (Book lice and Bark lice)
12. Order Mallophaga (Bird lice)
13. Order Anopleura (=Siphunculata) (Sucking lice)
14. Order Thysanoptera (Thrips)
15. Order Hemiptera (Bugs, Hoppers, Cicadas, White flies, Scale insects)
16. Order Grylloblattoidea (Grylloblatta)
Division Endopterygota (=Holometabola)
Wings develop internally inside the pupal case. Larva is very different in appearance and habits from the adult, has simple eyes and there is a distinct pupal stage. There are 9 orders in this group.
1. Order Neuroptera (Ant-lions and lacewings)
2. Order Coleoptera (Beetles and Weevils)
3. Order Strepsiptera (stylopids)
4. Order Mecoptera (Scorpion flies)
5. Order Trichoptera (Caddis flies)
6. Order Lepidoptera (Butterflies and Moths)
7. Order Hymenoptera (Wasps, Bees, Ants, Sawflies)
8. Order Diptera (True flies, Mosquitoes, Sandflies, gnats)
9. Order Siphonaptera (Fleas)
For detail classification go to INSECT ORDERS