Black flies lay their eggs on aquatic plants or rocks. The larvae, which are almost bottle-shaped, stick to the ground sticking the body out in the water. On the head they have a special ‘brush-like’ mouth part that collects tiny organisms out of the water. Pupation occurs in a cocoon that is open at one end and is stuck on rocks or plants. There can be several generations of adult black flies, but usually they are most numerous in the spring, in May. Unlike midges, black flies fly several kilometers away from their breeding sites.