Black fly, anterior view

Over 1,800 species of black flies are known (of which 11 are extinct). Most species belong to the immense genus Simulium. Most black flies gain nourishment by feeding on the blood of mammals, including humans, although the males feed mainly on nectar. They are usually small, black or gray, with short legs, and antennae. Black flies transmit the parasitic nematode Onchocerca volvulus , which causes Onchocerciasis or 'River Blindness', a condition causing itching and swelling of the skin, and blindness. They are a common nuisance for humans, and many U.S. states have programs to suppress the black fly population. Width of image is 1 millimeter.