Dictionary.net

Insects are a class, Insecta, of 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. They have lived on earth for about 350 million years, as compared with less than 2 million for man. While insects are often commercially valuable and useful as scavengers, many species are harmful, causing enormous losses in agriculture and storage. Three orders, HEMIPTERA, DIPTERA, and Siphonaptera, are of medical interest in that they cause disease in man and animal. (From Dorland, 27th ed; Borror et al., An Introduction to the Study of Insects, 4th ed, p1)