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.
Pale white at first, the eggs darken as embryo matures. 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.