Coloured scanning electron micrograph (SEM) of the head of a black fly (Prosimulium caudatum) larva showing the prominent feeding combs. P. caudatum is a blood-sucking insect closely related to mosquitoes. The head possesses two compound eyes, short segmented antennae and skin piercing mouthparts. Black flies are considered a human pest in some areas of the US and Canada. Adult females of certain species are fierce biters, whereas others are strictly a nuisance by their presence around exposed skin areas. Female black flies require a blood meal; males feed mainly on nectar. Adult black fly females lay their eggs in slow moving waters. Larvae emerge from eggs and attach themselves to aquatic vegetation and rocks. Also shown is the prothoracic proleg that is used to pull or hold threads of silk, as well as to grasp the silk pad it forms on a substrate in moving water. Most black fly larvae are filter feeders. A head fan sweeps food material into the mouth. Larvae pass through six instar stages before reaching the pupal stage. Pupae are encased in a silken cocoon attached to vegetation or other objects in the water. Adults emerge from the pupal case through a slit in the pupal cuticle and float to the surface on a bubble of air. Black flies can transmit filarial worms to humans resulting in a disease called onchocerciasis, which cause blindness. They may also be potential transmitters of encephalitis. Magnification: x10 when shortest axis printed at 25 millimetres.