Insects (Class Insecta) are a major group of arthropods and the most diverse group of animals on the Earth, with over a million described species—more than half of all known living organisms —with estimates of undescribed species as high as 30 million, thus potentially representing over 90% of the differing life forms on the planet. Insects may be found in nearly all environments on the planet, although only a small number of species occur in the oceans, a habitat dominated by another arthropod group, the crustaceans.

There are approximately 2,000 praying mantis, 5,000 dragonfly species, 20,000 grasshopper, 82,000 true bug, 120,000 fly, 110,000 bee, wasp and ant, 170,000 butterfly and moth, and 360,000 beetle species described to date. Estimates of the total number of current species, including those not yet known to science, range from two million to fifty million, with newer studies favouring a lower figure of about six to ten million. Adult modern insects range in size from a 0.139 mm (0.00547 in) fairyfly (Dicopomorpha echmepterygis) to a 55.5 cm (21.9 in) long stick insect (Phobaeticus serratipes).Walker, T.J., ed. 2001. University of Florida Book of Insect Records, 2001. 1 The heaviest documented insect was a Giant Weta of 70 g (2½ oz), but other possible candidates include the Goliath beetles Goliathus goliatus, Goliathus regius and Cerambycid beetles such as Titanus giganteus, though no one is certain which is truly the heaviest.

The study of insects (from Latin insectus, meaning "cut into sections") is called entomology, from the Greek εντομον, also meaning "cut into sections".

Body structure

Insects possess segmented bodies supported by an exoskeleton, a hard outer covering made mostly of chitin. The segments of the body are organized into three distinctive but interconnected units, or tagmata; a head, a thorax, and an abdomen. The head supports a pair of sensory antennae, a pair of compound eyes, one to three simple eyes ("ocelli") and three sets of variously modified appendages that form the mouthparts. The thorax has six segmented legs (one pair each for the prothorax, mesothorax and the metathorax segments making up the thorax) and two or four wings (if present in the species). The abdomen (made up of eleven segments some of which may be reduced or fused) has most of the digestive, respiratory, excretory and reproductive internal structures.