Black fly (Simuliidae)
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Any member of several species of small dipterans, most of which bite or annoy humans. Midges are also sometimes called gnats. In North America the name is also applied to the blackfly, fruit fly, and other small flies that hover about the eyes of humans and other animals.
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The males often assemble together in large mating swarms, particularly at dusk, called a "ghost".
Gnat larvae are mostly free-living and some are aquatic. Many feed on plants, though some are carnivorous. Larval plant feeders (e.g. the Hessian fly larva) cause root, stem, or 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.
Occasionally "gnat" is spelled "knat".