black fly

black fly

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.

A black fly (sometimes called a buffalo gnat, turkey gnat or white socks) is any member of the family Simuliidae of the Culicomorpha infraorder. They are related to the Ceratopogonidae, Chironomidae, and Thaumaleidae. There are over 1,800 known species of black flies (of which 11 are extinct). The majority of species belong to the immense genus Simulium. Like mosquitoes, to which they are related, most black flies gain nourishment by sucking the blood of other animals, although the males feed mainly on nectar. They are usually small, black or gray, with short legs and antennae. They are a common nuisance for humans, and many U.S. states have programs to suppress the black fly population. They spread several diseases, including river blindness in Africa (Simulium damnosum and S. neavei) and the Americas (Simulium callidum and S. metallicum in Central America, also S. ochraceum in Central and South America).

Role in Human Disease

The black fly is central to the transmission of the parasitic nematode Onchocerca volvulus, which causes Onchocerciasis, also known as river blindness. It serves as the larval host for the nematode and acts as the vector by which the disease is spread. Transmission of the parasite occurs through the bite of a black fly when feeding on human blood.

Regional effects of black fly populations

  • In the wetter parts of the northern latitudes of North America, including parts of Canada, New England and the Upper Peninsula of Michigan, black fly populations swell from Mid-May to July, becoming a nuisance to humans engaging in common outdoor activitites such as boating, camping and backpacking. They can also be a significant nuisance in mountainous areas.
  • In Canada, black flies are a scourge to livestock, causing weight loss in cattle and in some cases, death.
  • The Commonwealth of Pennsylvania, in the United States, operates the largest single black fly control program in North America. The program is seen as beneficial to both the quality of life for residents and to the state's tourism industry.
  • The Blandford Fly (Simulium posticatum) in England was once a public health problem in the area around Blandford Forum, Dorset, due to its large numbers and the painful lesions caused by its bite. It was eventually controlled by carefully targeted applications of Bacillus thuringiensis israelensis.


Eggs are laid in running water, and the larvae attach themselves to rocks. Breeding success is highly sensitive to water pollution. The larvae use tiny hooks at the end of the abdomen to hold on to the substrate, using silk holdfasts and threads to move or hold their place. They have foldable fans surrounding their mouths. When feeding, the fans expand, catching passing debris (small organic particles, algae and bacteria). Every few seconds, the larva scrapes the fan's catch into its mouth. Black flies depend on lotic habitats to bring food to them. They will pupate under water and then emerge in a bubble of air as flying adults. During emergence, they are often preyed upon by trout.

As adults, males feed on nectar while females also feed on blood. Some species in Africa can range as far as 40 miles from aquatic breeding sites in search of their blood meals, while other species have more limited range. Different species prefer different host sources for their blood meal, which is sometimes reflected in the common name for the species. They feed in the daytime, preferably when wind speeds are low. DEET based insect repellants may prove counter-productive and attract greater numbers of black flies; permethrin products designed for ticks are effective but can only be applied to clothing, limiting their utility.

Black flies are univoltine, which means that they spend the winter in the larval stage, often under the ice, where they slowly mature.

The Canadian Shield is characterized by an abundance of lakes and swift-flowing streams and hence offers optimum conditions for black flies to lay their eggs. The Canadian Shield is notorious for the abundance of black flies in the summertime.

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