Black Flies are small, ranging in size from 5 to 15mm, and black or grey in colour. They breed exclusively in running water. Because of their ability to filter dissolved organic matter and make it available to other species in the food chain, they are a keystone species in the ecology of water bodies. However, this trait also makes Black Flies very susceptible to both organic and inorganic pollution in our waterways, making them good indicator species for the ecological condition of fresh water streams and rivers.
Black Fly species vary in their preferred host for a blood meal, some preferring humans and others preferring birds and other mammals. Black Fly bites can be very painful, with itching and swelling generally occurring at the bite site. Intense feedings can cause a condition known as Black Fly Fever, with headache, nausea, fever, swollen lymph nodes and aching joints being the most common symptoms. In certain areas of the world, such as Africa and mountainous regions of South and Central America, Black Flies can transmit a parasitic nematode worm that can infect humans and cause a disease known as onchocerciasis or river blindness.
Black Flies are active only during the day, with peak activity around 9:00 to 11:00 AM and again from 4:00 to 7:00pm. They tend to be most active on humid, cloudy days and just before storms.