black fly, name for any of the flies of the family Simuliidae. The black fly is about 1/8 in. (3.2 mm) long and has large eyes, short legs, a stout, humped back, broad gauzy wings, and piercing-sucking mouthparts. The female inflicts a painful bite, sucking the blood of birds and mammals, including humans. Livestock and other large mammals may be bitten to death by swarms of black flies; the black fly problem of some subarctic regions is so severe as to make human settlement impossible. Some tropical African and American species carry the larvae of roundworms that in human hosts cause swellings of the skin and eyes and sometimes blindness. The eggs of black flies are commonly laid in masses on wet rocks, logs, and plants; the larvae live in fast flowing water, clinging to rocks by means of anal sucking disks and straining out organic matter by fanlike head organs. Pupation occurs underwater; the pupa accumulates a bubble of air in its case, enabling it to rise to the surface and emerge when mature. The Adirondack black fly, Simulium hirtipes, the white stockinged black fly, S. venustum, the buffalo gnat, S. pecuarum, and the turkey gnat, S. meridionale, are common species. Black flies are classified in the phylum Arthropoda, class Insecta, order Diptera, family Simuliidae. See insect.