There are all sorts of biting flies - we have some that look a lot like, but are a little smaller than, the usual housefly; they show up in the late summer (August or so) and are very persistent and annoying. Earlier in the season the tiny black flies will take a chunk out of your skin without you noticing it until you discover blood running down your arm (or wherever - they seem to particularly like my eyelids)...
What to do about them? Cover up with clothing.
