a blood-sucking dipterous fly, of the genus culex, undergoing a metamorphosis in water. the females have a proboscis armed with needlelike organs for penetrating the skin of animals. these are wanting in the males. in america they are generally called mosquitoes. see mosquito.-any fly resembling a culex in form or habits; esp., in america, a small biting fly of the genus simulium and allies, as the buffalo gnat, the black fly, etc.