gnat, any member of several species of small flies that bite and annoy humans. Several nonbiting insects, such as the midges, which resemble mosquitoes, are also sometimes known as gnats. In North America the name is often applied to the black fly, midge, fungus gnat, biting midge, fruit fly (qq.v.), and other small flies that hover about the eyes of humans and other animals. In Great Britain the name usually refers to mosquitoes or less commonly to crane flies (family Tipulidae).