A fly smaller than a fingernail can steal your sight
Black flies breed only in running water, yet adult females of some African species travel up to forty miles for blood meals. Their bites spread onchocerciasis—river blindness—caused by a nematode transmitted through saliva. Pennsylvania runs North America's largest control program; England's Blandford fly once sent bite victims to hospital.
Over 2,200 named species belong to family Simuliidae; more than 1,800 are in genus Simulium. Adults are small, dark, with short legs and antennae. Males feed on nectar; most females also require blood from birds or mammals to produce eggs. Eggs are laid in flowing water where larvae anchor to rocks with abdominal hooks and silk threads, filtering debris with expandable mouth fans. They pupate underwater and emerge in air bubbles—often becoming trout food.
Larvae depend on lotic habitats because currents deliver food. Research at Portsmouth University found their midguts create highly acidic conditions hosting cellulose-metabolizing bacteria—symbiosis that converts indigestible plant material into sugars neither insect nor stream alone could provide. Females bite by day in calm wind, stretching skin with labral teeth then abrading with mouthparts while anticoagulant saliva numbs the wound. Reactions range from itching to black fly fever with headache, fever, and swollen lymph nodes.
Simulium species vector Onchocerca volvulus, the nematode causing river blindness in Africa and the Americas. Mature adults can disperse hundreds of miles on wind, complicating control. Populations surge across northern North America from late April through July, plaguing campers and cattle—one species's saliva can trigger anaphylactic shock in livestock. Pennsylvania's statewide program protects tourism and quality of life; Dorset's Blandford fly was tamed with targeted Bacillus thuringiensis israelensis. Repellents with DEET, picaridin, or IR3535 help, but heavy clothing and head nets remain essential when swarms are unavoidable.
Source: Black fly
