From Envirowiki

An insect is a type of animal of the class "insecta". Insects have 6 legs (although some, such as catepillars, may have extra pseudolegs), and usually have a whole or partial exo-skeleton (they are invertebrates). Insect make up a large part of the world's biodiversity, and the vast majority of insect species are undocumented. Insects live in pretty much every ecosystem (except perhaps ocean floor sulphur vents)