Any member of the class Insecta, the largest arthropod class, including nearly 1 million known species (about three-fourths of all animals) and an estimated 5–10 million undescribed species. Insect bodies have three segments: head, thorax (which bears three pairs of legs and usually two pairs of wings), and many-segmented abdomen. Many species undergo complete metamorphosis. There are two subclasses: Apterygota (primitive, wingless forms, including silverfish and bristletails) and Pterygota (more advanced, winged or secondarily wingless forms). The approximately 27 orders of Pterygota are generally classified by wing form: e.g., Coleoptera (beetles), Diptera (dipterans), Heteroptera (bugs). Insects are found in almost all terrestrial and freshwater and some marine habitats.