The larva has a swollen posterior end bearing a circlet of hooks that grips a silk pad spun onto the substrate. Held in place, it turns its body into the flow and deploys the cephalic fans, arrays of fine rays that strain bacteria, algae and organic particles from the water. The fans are drawn through the mouthparts to clear them, and the process repeats continuously.
Because the fans need current, black flies breed only in running water, which is why the family is absent from ponds and why control has historically targeted river reaches rather than standing pools. A larva can shift position on a drag line of silk, letting the current carry it downstream and then hauling itself back.
Adult females of biting species are pool feeders, cutting the skin with short mouthparts. In the tropics some species transmit Onchocerca volvulus, the filarial worm behind river blindness, which is the reason the group has been the target of large control programmes.