Black Fly

Black flies make up the family Simuliidae, with more than 2,200 species found worldwide except Antarctica. Their larvae are unusual among insects for requiring flowing, well-oxygenated water, attaching themselves to rocks and submerged vegetation in streams and rivers rather than settling in still ponds.

Because larvae need clean, fast-moving water, black fly populations are often used as an indicator of good stream water quality, and their presence tends to track closely with healthy river ecosystems.

Adult black flies are best recognized by their humped, "buffalo" profile, a stocky build in which the thorax arches noticeably over the head, giving them a distinctive silhouette compared to other small biting flies.

• Small, stout body 1.5-5 mm long
• Distinctive humpbacked profile with the thorax arched over the head, giving the "buffalo gnat" nickname
• Black to dark gray body color
• Clear, unmarked, broad wings
• Short antennae and short legs relative to body size
• Lookalikes: other small biting flies lack the pronounced humped thorax that is diagnostic of black flies

Black flies occur worldwide except Antarctica, with larvae attached to rocks and vegetation in clean, fast-flowing streams and rivers. Adults are typically found near their breeding streams, especially in forested or mountainous temperate and subarctic regions, with populations peaking in spring and early summer.

Females of most species require a blood meal from mammals or birds to develop eggs and are persistent daytime biters, often forming dense swarms around the head and exposed skin near flowing water; males instead feed on nectar and plant sugars. Larvae are filter feeders, using fan-shaped mouth brushes to strain algae, bacteria, and organic particles from the current while anchored to submerged surfaces with silk pads.

Eggs are laid on wet rocks, vegetation at the water's edge, or scattered directly onto flowing water. Larvae attach themselves to submerged surfaces using a silk pad and hooked prolegs, passing through several instars while filter-feeding from the current, typically over two to four weeks in warm conditions. Pupation occurs underwater within a silk cocoon, after which the adult emerges and rises to the surface inside an air bubble. Many species produce multiple generations per year, while others overwinter as eggs or larvae in colder climates.