These are very interesting flies!
They remind me of a head shot that Charlie Krebs did about a year ago:
here. I wonder if they're the same kind? But I see we never got an ID on Charlie's, so that doesn't help much.
Hhmm... OK, on quick browse through BugGuide, let me suggest perhaps a couple of these fruit flies,
tribe Xyphosiini in family Tephritidae. But that's just a guess from general appearance -- I don't really know what the distinguishing characteristics are.
--Rik