gnat, common name for any one of a number of small, fragile-looking two-winged flies of the suborder Nematocera, order Diptera, which includes the families Tipulidae (crane flies), Bibionidae (hairflies), Ceratopogonidae (biting midges), Chironomidae (true midges), Cecidomyidae (gall midges), Simuliidae (black flies), Culicidae (mosquitoes), and others. They often assemble together in large mating swarms. In England mosquitoes are called gnats; in the United States it is chiefly the smaller forms of Diptera, especially irritating because of their great numbers and their vicious biting habits, that are referred to as gnats. All gnats have long, hairlike antennae, which are particularly well-developed in the males. Gnat larvae are free-living, most feeding on plants. Larval plant feeders, e.g., the Hessian fly larvae, cause root, stem, and leaf galls to be formed by the host plant. Some species of fungus gnats (families Mycetophilidae and Sciaridae) are very common pests of mushrooms and roots of potted plants in homes and greenhouses. One group of dipterans (family Chloropidae) of the suborder Cyclorrhapha are commonly called eye gnats, or eye flies. Gnats are classified in the phylum Arthropoda, class Insecta, order Diptera.

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