5 to 15 mm long, whitish-brown to blackish and with a distinct head and anterior proleg. The head has slender antennae and two brush-like structures called cephalic fans. If removed from the cocoon, the wing pads, legs, and other features of the adult fly can be discerned although these structures are closely appressed to the body. However, some species occur in slow moving streams, in swamps, or in rivers. Female black flies feed upon man, livestock, poultry, wild birds, and mammals.
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.