These are probably the most successful group of all animals with around one million named species making up 3/4 of all described animal species. It has been estimated that the actual number of insect species could be as much as 5 to 10 million.

Insects are six-legged arthropods with bodies segmented into three parts, head, thorax, and abdomen.   The word “insect” comes from the Latin word insectum, meaning “with a notched or divided body”. They also have a pair of compound eyes and antennae.   Most adults also have two pairs of wings.

They are usually subdivided into the two subclasses Aptrygota (wingless insects) and Pterygota (winged insects), the latter being further subdivided into Exopterygota (external wing development, incomplete or gradual metamorphosis, no pupal stage, nymph) and Endopterygota (internal wing development, complete metamorphosis, a pupal stage, larvae). The Encyclopedia Britannica has divided the subclass Pterygota into further infraclasses and superorders. However, the Royal Entomological Society has divided them into 5 subclasses.

Here is a list using the two subclasses with their orders, as listed in iNaturalist, sorted alphabetically, with the number of species listed in Catalogue of life* (2019) in parenthesis:

1. Aptrygota — Wingless insects
   1. Archaeognatha — Bristletails
   2. Zygentoma — Silverfishes
2. Pterygota — Winged and once-winged insect
   1. Blattodea — Cockroaches and termites (7,568)
   2. Coleoptera — Beetles (315,888)
   3. Dermaptera — Earwigs (1,894)
   4. Diptera — True flies (153,387)
   5. Embiidina or Embioptera — Webspinners (398)
   6. Ephemeroptera — Mayflies (3,341)
   7. Hemiptera — True bugs (82,161)
   8. Hymenoptera — Ants, bees, and wasps (117,847)
   9. Lepidoptera — Butterflies and moths (149,821)
   10. Mantodea — Mantises (2,490)
   11. Mecoptera — Scorpionflies, hanging flies, and allies (684)
   12. Megaloptera — Alderflies, dobsonflies, and fishflies (386)
   13. Neuroptera — Antlions, lacewings, and allies (5,917)
   14. Notoptera — Ice and rock crawlers (34*)
   15. Odonata — Dragonflies and damselflies (5,912)
   16. Orthoptera — Grasshoppers, crickets, and katydids (27,344)
   17. Phasmida — Stick-insects (3,284)
   18. Plecoptera — Stoneflies (3,691)
   19. Psocodea or Psocoptera — Booklice, bark lice and parasitic lice (10,963)
   20. Raphidioptera — Snakeflies (253)
   21. Siphonaptera — Fleas (2,047)
   22. Strepsiptera — Twisted-winged insects (602)
   23. Thysanoptera — Thrips (6,222)
   24. Trichoptera — Caddisflies (11,513)
   25. Zoraptera — Angel insects (43)

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*For an updated list see Catalogue of Life Insecta page.

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