any member of a family of insects in the fly order, Diptera, that are small, dark coloured, and commonly found in great numbers around ponds, streams, and the seashore. Most larvae are aquatic, and some species can tolerate highly saline or alkaline waters—such as Ephydra riparia, a species that inhabits the Great Salt Lake in Utah. Another interesting species is the carnivorous petroleum fly (Helaeomyia petrolei), which lives and breeds in pools of crude petroleum and feeds on trapped insects. At one time, Indians in the western United States gathered the aquatic larvae of shore flies for food.