Kingdom: Animalia
Phylum: Arthropoda
Clade: Pancrustacea
Subphylum: Hexapoda (six legged)

Insects (from Latin insectum, a calque of Greek ἔντομον [éntomon], "cut into sections") are a class of invertebrates within the arthropod phylum that have:

  •  a chitinous exoskeleton, 
  • a three-part body (head, thorax and abdomen), 
  • three pairs of jointed legs,
  •  compound eyes and 
  • one pair of antennae. 


A cricket or grasshopper insect. image: R.Conan-Davies

They are among the most diverse groups of animals on the planet, including more than a million described species and representing more than half of all known living organisms.  The number of extant species is estimated at between six and ten million, and potentially represent over 90% of the differing animal life forms on Earth. 

The anatomy of an insect

A- Head
, B- Thorax, 
 C- Abdomen 1. antenna 2. ocelli (lower) 3. ocelli (upper) 4. compound eye 5. brain (cerebral ganglia) 6. prothorax 7. dorsal artery 8. tracheal tubes (trunk with spiracle) 9. mesothorax 10. metathorax 11. first wing 12. second wing 13. mid-gut (stomach) 14. heart 15. ovary 16. hind-gut (intestine, rectum & anus) 17. anus 18. vagina 19. nerve chord (abdominal ganglia) 20. Malpighian tubes 21. pillow 22. claws 23. tarsus 24. tibia 25. femur 26. trochanter 27. fore-gut (crop, gizzard) 28. thoracic ganglion 29. coxa 30. salivary gland 31. subesophageal ganglion 32. mouthparts. image: wikipedia

Insects are just about everywhere

Insects may be found in nearly all environments, although only a small number of species reside in the oceans, a habitat dominated by another arthropod group, crustaceans.

Insect lifecyles

The life cycles of insects vary but most insects hatch from eggs. Insect growth is constrained by the inelastic exoskeleton and development involves a series of molts. The immature stages can differ from the adults in structure, habit and habitat, and can include a passive pupal stage in those groups that undergo complete metamorphosis. Insects that undergo incomplete metamorphosis lack a pupal stage and adults develop through a series of nymphal stages.

typical lifecyle of butterfly. image: wikipedia

The higher level relationship of the hexapoda is unclear. Fossilized insects of enormous size have been found from the Paleozoic Era, including giant dragonflies with wingspans of 55 to 70 cm (22–28 in). The most diverse insect groups appear to have coevolved with flowering plants.

Insects have diverse ways of moving

Adult insects typically move about by walking, flying, or sometimes swimming. As it allows for rapid yet stable movement, many insects adopt a tripedal gait in which they walk with their legs touching the ground in alternating triangles. Insects are the only invertebrates to have evolved flight. 

Many insects spend at least part of their lives under water, with larval adaptations that include gills, and some adult insects are aquatic and have adaptations for swimming. Some species, such as water striders, are capable of walking on the surface of water. 

Some insects are solitary some are social and caring

Insects are mostly solitary, but some, such as certain bees, ants and termites, are social and live in large, well-organized colonies. Some insects, such as earwigs, show maternal care, guarding their eggs and young. Insects can communicate with each other in a variety of ways. 

Insects communicate in various ways

Male moths can sense the pheromones of female moths over great distances. Other species communicate with sounds: crickets stridulate, or rub their wings together, to attract a mate and repel other males. Lampyridae in the beetle order Coleoptera communicate with light.

Human and insect relations


Humans regard certain insects as pests, and attempt to control them using insecticides and a host of other techniques. Some insects damage crops by feeding on sap, leaves or fruits. A few parasitic species are pathogenic. Some insects perform complex ecological roles; blow-flies, for example, help consume carrion but also spread diseases. 

Essential and helpful relations

lnsect pollinators are essential to the life-cycle of many flowering plant species on which most organisms, including humans, are at least partly dependent; without them, the terrestrial portion of the biosphere (including humans) would be devastated. 

Many other insects are considered ecologically beneficial as predators and a few provide direct economic benefit. Silkworms and bees have been used extensively by humans for the production of silk and honey, respectively. In some cultures, the larvae or adults of certain insects are a food-source for humans.

The study of insects is called entomology and is broadly part of zoology

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