[4] The fans expand when feeding, catching passing debris (small organic particles, algae, and bacteria).

This symbiotic relationship indicates a specific adaptation, as fresh-flowing streams could not provide sufficient nutrition to the growing larva in any other way.

Simulium, the type genus, is the most widespread and is a vector for several diseases, including river blindness.

Mature adults can disperse tens or hundreds of miles from their breeding grounds in fresh flowing water, under their own power and assisted by prevailing winds, complicating control efforts.

Swarming behavior can make outdoor activities unpleasant or intolerable, and can affect livestock production.

[11] Even non-biting clouds of black flies, whether composed of males or of species that do not feed on humans or do not require a blood meal before egg laying, can form a nuisance by swarming into orifices.

Biting flies feed during daylight hours only and tend to zero in on areas of thinner skin, such as the nape of the neck or ears and ankles.

[14] However, given the limited effectiveness of repellents, protecting oneself against biting flies requires taking additional measures, such as avoiding areas inhabited by the flies, avoiding peak biting times, and wearing heavy-duty, light-colored clothing, including long-sleeve shirts, long pants and hats.

When black flies are numerous and unavoidable, netting that covers the head, like the “bee bonnets” used by beekeepers, can provide protection.

[15] Black flies are central to the transmission of the parasitic nematode Onchocerca volvulus which causes onchocerciasis, or "river blindness", which is endemic in parts of South America, Africa, and the Arabian Peninsula.