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)