Handbook of Insect Biology

Insects are numerous usually small arthropod animals of the class Insecta,
having an adult stage characterized by three pairs of legs and a body segmented
into head, thorax, and abdomen and usually having two pairs of wings. Insects
include the flies, crickets, mosquitoes, beetles, butterflies, and bees. Like other
arthropods, an insect has a hard outer covering, or exoskeleton, a segmented
body, and jointed legs. Adult insects typically have wings and are the only
flying invertebrates. The body of the typical adult insect is divided into three
distinct parts, the head, thorax, and abdomen. The exoskeleton is composed
of a horny substance called chitin.