Black Flies
This common name refers to only one family.
These are pretty much always called Black Flies.
Black flies are not usually regarded as important trout fare, but scientific studies of some rivers have shown them to make up the majority of the trout's diet. Such places are few and far between, but anglers should be aware of the possibility and keep a lookout for high concentrations of the larvae.
The adults are nasty, annoying, biting flies.