There were scores of these medium sized flies (about 9mm) in the shady vegetation along a woodland trail, on my walk today. They have an interesting biology, at least the females do. Here is what Bug Guide says about them:

"adult females congregate in a swarm above vegetation near water around sunset, and inflate abdominal sacs in an attempt to fool males into thinking the swollen abdomens are full of ripe eggs. The hairy legs are held alongside the abdomen in flight, supposedly to accentuate abdominal size. Males hunt small insects and bring the dead prey as "nuptial gifts" to females in exchange for a chance to mate. Males prefer to mate with the fattest and hairiest females, but the cost of being hairy is an increased chance of being caught in spider webs."