Insect
Insect, invertebrate animal of the class Insecta of the phylum Arthropoda. 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 head bears three pairs of mouthparts, one pair of compound eyes, three simple eyes (ocelli), and one pair of jointed sensory antennae. The thorax is divided into three segments, each with a pair of jointed legs, and bears two pairs of wings. The abdomen has posterior appendages associated with reproduction. The exoskeleton is composed of a horny substance called chitin. Insects breathe through a complex network of air tubes (tracheae) that open to the outside through a series of small valved apertures (spiracles) along the sides of the body. In chewing insects the digestive system includes a muscular gizzard that is lac...