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Many dance flies present prey as nuptial gifts as a part of their mating rituals, but as Powell and Hogue write in California Insects, "The strangest habits are exhibited by a few species in which males secrete material from the digestive tract which is formed into a frothy, white ball or "balloon". This balloon either entraps a small prey or is substituted for prey in the courtship gift-giving sequence" (p. 161). This balloon clearly has prey within, perhaps some kind of hemipteran.

Ron and I were pretty flabbergasted as we watched these two engage in mid-air copulation, their aerial control so perfect that I was able to get this picture even though each fly couldn't have been more than 7 mm long.

Jack Owicki has this great BG photo that spawned an even greater discussion: http://bugguide.net/node/view/267952/bgimage.

I suspect this is Empis and not another genus in Empidinae due to the balloon-forming behavior and the long proboscis, but I'd love correction! Habitat was very close to a running creek, vegetation was riparian, some willow, but mostly oaks and maples.