Animated coloured scanning electron micrograph (SEM) of Black fly larva (Simulium hippovorum). Note the ribbon of silk coming from the mouth area. All known species of black flies produce silk for mooring lines for anchorage to substrates in flowing water. A pair of silk glands run the length of the body, the strands of silk emerge from a silk duct below the mouth and are cut and guided by a series of teeth on the mandible and hypostoma. 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. Black flies can transmit filarial worms to humans resulting in a disease called onchocerciasis, which cause blindness.