These critters show up regularly at "Josh's Diner", my butterfly bait station outside my office. They are apparently sterile flies released by the USDA; they are grown in a red-dyed medium, and when the adult flies emerge from their pupal cases, a red dot remains visible on their foreheads. My next-door neighbor, Dr. Don Vacek of the USDA, identified these flies for me and explained the system for marking the sterile flies.