Their called Stable Flies.
They look like a smaller version of the common house fly,with light tan stripes on their rear end.
They only show up on calm days,and only when your wearing shorts.
They bite right through socks.
Stable Flies
Jeffrey Hahn, Assist. Extension Entomologist
Stable fly. :Jeff Hahn:
Whether you are outside at your favorite lake cabin or sitting in your backyard in town, you probably have experienced a problem with flies biting at or near your ankles. These flies, similar in appearance to house flies, are known as stable flies. Stable flies, sometimes called biting house flies or ankle-biters, are about 1/4 - 3/8th and are gray with four longitudinal black stripes on their thorax.
However, while house flies have sponging mouthparts for sopping up liquid food, the mouthparts of stable flies are modified into a long, sharp bayonet-like proboscis. They feed on the blood of not only people but on many other mammals, including dogs, cattle, and horses. They are very persistent and very annoying.