Members of the family, Simuliidae (Diptera) of which there are more than 1,000 known species, are small flies, 1-5 mm in length. The common name, black fly, is somewhat of a misnomer since many species are gray and others are light tan to yellow in color.

Adults are typically robust with a characteristic arched or humped prothorax giving rise to the common name,"buffalo gnat". The family Simuliidae is worldwide in distribution, extending from the tropics to the Arctic Circle.

Both sexes feed on the nectar of flowers; only the female requires a blood meal for ovarian development. Although a given species may prefer a particular animal host, most will readily feed upon other host species as well.

Black flies attack man and a wide variety of domestic and wild animals and birds; others feed only on cold-blooded animals. Many important pest species belong to the genus Simulium.

Females deposit 150-500 creamy-white eggs that darken until they are almost black just prior to hatching. The eggs usually are deposited in masses on some convenient object in or near the edge of flowing water.

The water may be a slow-moving stream, but most species prefer rapidly flowing water. In rare cases, breeding may occur in marshy areas. The time required for hatching varies with the species and may be 3-5 days or as long as 30 days at low temperatures.

Larvae most often are found just beneath the surface of rapidly flowing water. Most species feed on small animals; such as protozoa and crustaceans; or plants such as algae, by straining particulate matter from water flowing by them with the aid of mouth brushes. Some species feed in the silt or on submerged surfaces.

Since the gill filaments of larvae extract oxygen directly from the water, they do not come to the surface to breath as do mosquito larvae. Duration of larval development, like the egg stage, varies with species and temperature; it may range from 10-14 days to 7-10 weeks.

Some species overwinter as larvae; in the arctic and other far northern areas, winter is usually passed in the egg stage. The last larvae instar spins a reddish-brown, basket-like cocoon in which pupation occurs. These cocoons are attached to slightly submerged objects such as rocks, logs, roots and other debris. Pupae also possess respiratory filameters with which they remove dissolved oxygen from the water.

The pupal period varies from 4-5 days to as long as 3-5 weeks, depending on water temperature and species. Adults emerge from the pupal case, rise to water's surface, unfold their wings and fly away.

Mating usually occurs shortly after the initial flight. The complete life cycle, from egg to adult, varies from 6-15 weeks and the number of generations per year ranges from one to six, depending on species and climatic conditions.

There are numerous references of livestock losses along river basins in the United States, as well as other countries, resulting from black fly attacks.

Large numbers of cattle, horses, mules, hogs, turkeys, chickens, sheep, dogs and cats, as well as wild animals have been killed. Death usually occurs as a consequence of an acute toxemia, caused by a vast number of bites, or as a result of anaphylactic shock. Weakness due to blood loss may also be a contributing factor to animal loss.

The most common disease transmitted by buffalo gnats in the United States is a leucocytozoon infection of turkeys. This disease, caused by a protozoan, is fatal to turkeys. A similar disease also occurs in ducks.

In other parts of the world, these flies serve as intermediate hosts for several nematode parasites of man and some domestic animals.
Black fly control is difficult due to the extent and number of variable larval breeding sources. Approved insecticides have been applied to rivers, streams and canals in which breeding occurs. However, pupae are not destroyed and the more effective chemicals may produce undesired environmental effects.
Individual animals may be stabled during the day when populations are abundant, but this practice is not practical for large herds. Fly repellents applied to the chest, belly and ears are somewhat effective but require daily application. Muslin or cotton bags fitted over horses' ears may be used to prevent fly entry of those species attacking these areas.