entomology, study of insects, an arthropod class that comprises about 900,000 known species, representing about three fourths of all the classified animal species. Insects are studied because of their importance as pollinators for fruit crops; as carriers of bacterial, viral, and fungal diseases; as parasites of humans or livestock; as destroyers of economically important plants; or as predators of other destructive insects. The role of insects in ecosystems and their control by insecticides or by biological methods are studied in ecology. Some insects such as the fruit fly, Drosophila, are used in the laboratory to study genetics; others are used to study behavior and physiology. The ability to increase productivity of insect populations that supply commercially important products such as dyes, silk, and honey and the deliberate introduction of insect diseases into populations of insect pests involves knowledge of microbiology and biochemistry as well as entomology.