The Black Fly

Common Name:
Black Flies
Scientific Name:


The common name of black flies comes from their typical black color. In various areas, some species are called buffalo gnats, others turkey gnats. Their pest status comes from the very painful bite of the females, but in Mexico, Central and South America, and Africa some species are vectors of a filarial worm which causes onchocerciasis or river blindness. They are worldwide in distribution. Although found throughout the United States, about 143 species, they are more common in northern areas, especially around areas of fast-moving water such as mountain streams and rivers.


Adults about 1/32-1/4" (1.2-5.5 mm) long; body robust, somewhat humpbacked. Color dark, usually black or brown but sometimes reddish brown, gray, yellow, or orange. Head with ocelli absent, compound eyes separated (dichoptic) in female but usually touching (holoptic) in male. Antennae short, stout, segments beadlike, usually 1 1 -segmented (range 9-1 1). Wings broad at base, narrowing toward tip; front veins heavy, rest weak, with thickened part of costal vein extending beyond 3/4's way to but ending before apex/tip.

Mature larvae up to about 3/8-5/8" (1 0-1 5 mm) long; color whitish to brown; narrowest in their midregion; and with a single thoracic proleg. Larvae aquatic, usually in swift- flowing water.


  1. Sand flies (Ceratopogonidae) with thickened costal vein usually ending 1/2-2/3's way to wing tip, antennae longer, 15-segmented, of small globular segments, usually with whorls (circles) of hairs (setae).

  2. Mosquitoes (Culicidae) have wing with costa continued around apex, wings with scales along veins and wing margin, and antennae 12-16-segmented.

  3. Moth flies (Psychodidae) have wing with costa continued around apex, wings densely covered with hairs, and antennae 12-16- segmented.


  1. Southern buffalo gnat, Cnephia pecuarum (Riley). Length about 1/16" (2.5 mm); color gray; attacks humans and livestock, a severe pest in Mississippi River system before flood control measures implemented; found in Illinois, Indiana, Ohio, Pennsylvania, New Jersey, Maryland, southeastern U.S., and Texas.

  2. Turkey gnat, Simulium meridionale Riley. Length about 1/16" (2 mm); color grayish or brownish black; attacks turkeys and other fowl, may cause choleralike symptoms in poultry; found throughout the United States.

  3. Simulium venustrum Say. Length about 1/16" (2.5 mm); color dark with white bands on tibiae; readily attacks humans; widespread, especially in northern states.

  4. Simulium vittatum Zetterstedt. Length about 1/8" (2-3 mm); color dark gray to velvety black; attacks domestic animals; widespread in the United States.


Females of pest and disease-vector species require a blood meal in order to lay viable eggs. Eggs are typically laid in batches of 200-500 on objects partially submerged in turbulent water or objects washed or sprayed by such water, but some species merely drop their eggs in turbulent water, and a few species breed in irrigation canals and ditches. Eggs usually hatch in 3-7 days but those laid in late summer or autumn overwinter first. Larvae attach themselves to objects in the water by means of a toothed anal sucker after covering the attachment site with silk from their salivary glands. Larvae feed on organic debris, diatoms, and bacteria filtered from the water. They obtain oxygen via retractile anal respiratory gills. Under favorable conditions the larva passes through 5-9 (usually 7) instars in about 30 days for eggs which hatch in the spring or summer. The pupal cocoon is also attached to submerged objects with oxygen being obtained via pupal gills which trail into the water from the head-thoracic region. Adults typically emerge in 2-6 days during the daytime. They are encased in an air bubble as they leave the cocoon which propels them upward to the surface and into the air. Mating usually takes place in swarms, but occurs on the ground for a few species. Adults usually live less than 3 weeks. Depending on the species, there may be 1-5 generations per year. Species with more than 1 generation overwinter as larvae.

Black flies are particularly severe pests in the north temperate and sub-arctic areas where they occur in huge swarms. Here they make life rather miserable for humans, domestic animals, and wildlife. There are records of hundreds to thousands of livestock being killed, probably by toxaemia or anaphylatic shock from bites, blood loss, and/or breathing problems due to inhalation of large numbers of flies. Lower levels of biting cause economic loss in livestock due to agitation. Black flies are also important vectors of disease organisms. In Mexico, Central and South America, and Africa, some species vector the filarial worm Onchocerca volvulus O'Neil which causes onchocerciasis or river blindness in humans. They also vector various protozoan diseases to poultry in the United States.

Bites are very painful. Black flies feed from the pool of blood exuding from the bite/wound, requiring 4-5 minutes for a blood meal. Bites occur on exposed skin, where clothing fits snugly, and also under clothing. The bite typically has a small central red spot surrounded by a pinkish.swolien area. Apparently a toxin is injected. Symptoms typically include headache, fever, nausea, and adenitis (inflammation of 1 or more lymph nodes). Bites become increasingly itchy and swollen, and remain irritating for several days.


The larvae develop only in fast-moving water where they attach to submerged objects. Here they feed by filtering the water through special combs which removes nutrients. The pupae also occur in fast-moving water. Upon emergence from the water adults fly off immediately. Both sexes feed on nectar but the female requires a blood meal for egg development. Depending on the species, acceptable water conditions for larval development range from torrential mountain streams to slow moving lowland rivers, and include irrigation canals and ditches.

Feeding is essentially diurnal, occurring during early morning and early evening hours in sunny areas. In shady areas, biting is more evenly distributed throughout the daytime. They do not bite late at night like mosquitoes and biting midges. Within structures, they do not bite even during the day. Adults of some species remain close to the larval breeding areas while others disperse at least 93 miles (150 km); they can be blown for miles by the wind. They enter structures accidentally and typically fly to windows where they crawl about until death. Adults of some species readily feed on humans while others prefer other mammals or birds, some species are generalists feeding on both mammals and birds while others are quite specific as to which mammals or birds are acceptable hosts. Adults of both sexes feed on nectar as an energy source. Most human pest species begin to emerge as adults in May and die off by early July.