Black flies get into the nose, eyes, ears and mouth and crawl into clothing when abundant. On cattle and horses, the ears seem to be the favorite feeding location. Flies lacerate the skin and suck blood. A strong anticoagulant in the saliva facilitates bleeding of the host. A lesion may form at the bite and the bitten area itches.
Black flies serve as vectors of human and animal diseases in some areas of the world.
An important protozoan parasite of poultry, Leucocytozoon simondi , is transmitted by black flies in the United States. Because of their distinct humpbacked shape, they are sometimes called buffalo gnats. The wings are broad and the antennae are about as long as the head. Male black flies have larger eyes than females. Some species have white markings.