Black flies, also known as buffalo gnats or turkey gnats, are small, stout-bodied insects belonging to the family Simuliidae. They are characterized by a distinctive arched or humped back. Where these insects live depends entirely on the stage of their life cycle, but they are found nearly everywhere that water flows. Their presence indicates a nearby aquatic habitat, as their entire immature phase is water-bound.
Worldwide Distribution of Black Flies
Black flies are one of the most widely distributed insect families, with over 2,000 recognized species found across the globe. Their distribution spans every continent except Antarctica, generally lacking populations only in the most extreme polar regions and true deserts. They are particularly numerous in temperate zones, such as North America, Europe, and Asia, where they are often a significant springtime nuisance.
The diversity of the Simuliidae family allows different species to adapt to almost every climate. Certain species in Central and South America and Africa are vectors for the parasitic nematode that causes river blindness, making them a public health concern in those tropical regions. However, the distribution of black flies is always restricted by the availability of the specific aquatic environment required for their young.
The Essential Aquatic Breeding Grounds
Black flies live in flowing water, which is a requirement for their larval and pupal stages. Unlike mosquitoes, black flies cannot breed in stagnant bodies of water. They are strictly confined to lotic systems, including small mountain streams, trickling creeks, large rivers, and artificial spillways.
The constant flow of water is necessary for two primary reasons: oxygenation and feeding. Larvae require high levels of dissolved oxygen, which the turbulence of moving water provides. The larvae are filter feeders, using specialized mouthparts called labral fans to strain microscopic organic particles out of the current for nutrition.
To remain in this fast-moving environment, the larvae attach themselves to submerged surfaces using a posterior suction cup and a silken thread. They anchor to rocks, vegetation, or debris, remaining stationary as the current delivers their food supply. While many species require clean water, some tolerate moderate urban pollution, but they are absent from severely impaired or heavily polluted sites.
Seasonal Activity and Adult Flight Range
The black fly season, when people encounter biting adults, usually begins in late spring or early summer in temperate regions. After spending their larval and pupal stages underwater, adults emerge and become terrestrial, moving away from the water source. Males feed only on nectar for energy, but females of most species require a blood meal to develop eggs, leading them to seek out humans and animals.
While some species are dispersed over vast distances by strong winds, most adult black flies stay relatively close to their natal stream. Their typical flight range is limited, usually spanning only a few miles (6 to 10 kilometers) from the water source where they emerged. Their presence in a backyard or park indicates a flowing water source nearby.
Adult activity is restricted to the daytime, often peaking in the morning and again in the late afternoon or early evening. The flies are attracted to the carbon dioxide emitted by their hosts, which is why they frequently swarm around the head and face. The adult stage typically lasts only a few weeks before the cycle ends until the following season.