The class Insecta, in the phylum ARTHROPODA, whose members are characterized by division into three parts: head, thorax, and abdomen. They are the dominant group of animals on earth; several hundred thousand different kinds having been described. Three orders, hemiptera; diptera; and siphonaptera; are of medical interest in that they cause disease in humans and animals. (From Borror et al., An Introduction to the Study of insects, 4th ed, p1)