In traditional classifications, a subphylum (or class) of arthropods comprising over a million known species (of an estimated 5.5 million species). They are distributed worldwide in nearly all terrestrial habitats. Ranging in length from 0.5 to over 300 mm, an insect’s body consists of a head, a thorax of three segments and usually bearing three pairs of legs and one or two pairs of wings, and an abdomen of eleven segments. The head possesses a pair of sensory antennae and a pair of large compound eyes, between which are three simple eyes (ocelli). The mouthparts are variously adapted for either chewing or sucking, enabling insects to feed on a wide range of plant and animal material. Insects owe much of their success to having a highly waterproof cuticle (to resist desiccation) and, in most species, wings—outgrowths of the body wall that confer the greater mobility of flight. Breathing occurs through a network of tubes (see trachea).

Most insect species have separate sexes and undergo sexual reproduction. In some, this may alternate with asexual parthenogenesis and in a few, males are unknown and reproduction is entirely asexual. In the wingless insects (subclass Apterygota) metamorphosis is slight or absent. In the winged insects (subclass Pterygota) the newly hatched young grow by undergoing a series of moults. In the more primitive exopterygotes (including the orders Dermaptera, Orthoptera, Dictyoptera, and Hemiptera) the young (called a nymph) resembles the adult. The more advanced endopterygotes (e.g. Coleoptera, Diptera, Lepidoptera, and Hymenoptera) undergo metamorphosis, in which the young (called a larva) is transformed into a quiescent pupa from which the fully formed adult emerges. Insects are of vital importance in many ecosystems and many are of economic significance—as animal or plant pests or disease vectors or beneficially as crop pollinators or producers of silk, honey, etc.

Some authorities regard the Hexapoda as a superclass comprising the classes Protura, Collembola, and Diplura, all of whose members have mouthparts that are enclosed in folds of the head; and Insecta, which have exposed mouthparts. Genetic and other molecular evidence now indicates that insects evolved from crustacean ancestors. Hence, the insects and crustaceans are now placed in the clade Pancrustacea, with a group of predatory crustaceans, the Remipedia, regarded as the closest relatives to all insects.

http://tolweb.org/tree?group=Insecta Overview of insect characteristics and phylogeny, with links to insect orders, at the Tree of Life web project