Thes flies were hyperkinetic, scurrying along the ground, apparently trying to take off while coupled. As they got close, they would try to climb my leg. Size estimated. See comments on identification--thanks. These are possibly Rachicerus fulvicollis, since that is the only species listed at North Carolina State University. What are probably the details of that very specimen are listed by Brimley, p. 329, noting it from Raleigh (about 30 miles from the site of this photo) in June (1).
Update. Identified below as R. obscuripennis.

Size estimated at about 12 mm for the female.