Black flies have an aquatic larval stage, and emerge as adult flies to reproduce.


Black flies, also known as "gnats," are true flies, in the order Diptera and the family Simuliidae. Adult black flies are very small, usually measuring about 3 mm or less. When compared to other biting flies, such as mosquitoes, black flies have a more robust, stout body, shorter legs, and fewer veins in their wings. However, many of these features aren't easily seen without magnification.


The primary nuisance black fly species in Maryland is Simulium jenningsi.

In 2013 we identified the species of black fly that formed nuisance swarms in Washington County as Simulium jenningsi, a species found as a nuisance pest throughout the Mid-Atlantic. As of 2016, we have collected S. jenningsi specimens from Washington, Frederick, Montgomery, and Prince George's Counties. We have also received complaints from residents in Garrett and Cecil Counties. 


Photo credit: Dave Penrose

Photo credit: Dave Penrose

Black fly larvae are aquatic filter feeders.

The immature stages of all black flies are small, often less than 1 cm in length, have no jointed appendages, and live on the bottoms of streams and rivers. They produce silk from their mouthparts, which they use to attach themselves to substrates like plants, rocks, and even trash. The larvae require flowing waters to develop, and won't be found in ponds. Although black fly larvae of many species can be found in small streams, the larvae of S. jenningsi are found in streams and rivers greater than 6 meters in width. In western Maryland, the Potomac River serves as their primary larval habitat.


Black fly adult females feed on blood.

After the larvae have completed their development underwater, they enter a pupal stage and undergo metamorphosis, transforming into the winged adult stage. The adults are highly mobile, and some species may regularly travel 15 to 50 km from where they leave the stream to feed and find water to lay eggs. Adults feed only on liquids such as nectar, and have a longevity of 10-35 days. Female black flies require a blood meal from a warm-blooded vertebrate (such as birds, deer, or humans) to reproduce.

Photo credit: Rich Merritt

Photo credit: Rich Merritt


Adult female black fly feeding on human host.

Black flies and humans. 

There are approximately 165 species of black flies scattered all throughout North America, but only a subset of those species are a pest to humans. In tropical areas, black flies are known to transmit diseases caused by nematode worms, but there are no known human diseases transmitted by black flies in the United States. The voracious feeding of black fly adults can be an extreme nuisance to humans, and can cause bleeding, itching, and in some cases a rash and swelling. Feeding females can also cause problems for livestock and wildlife when swarms are very large.