Maine is notorious for its black flies. Black flies are generally small, black or gray, with short legs and very short antennae. They belong to a family of flies called the Simuliidae family, of which there are four genera containing species that feed on people. There are additional species that prefer instead to feed either on other mammals, or else birds. Of those that require blood, it is only the females that bite (as is the case with mosquitoes as well), with the males feeding principally on nectar. Black flies need running water in which to breed. By contrast, mosquitoes require much more stagnant water such as ponds and puddles. Some black fly species live in large, fast-flowing streams, while others prefer small, sluggish waterways. However, almost any kind of stream can serve as a home to some particular species of black fly. Some species can even breed successfully in irrigation ditches. Breeding success of black flies is highly sensitive to water pollution, making them an important indicator species for assessing the quality of the water in a given river or stream. But, they are a common nuisance overall, and so many states have programs in place to suppress their populations.
Additional Information: Black Flies (University of New Hampshire Cooperative Extension)