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 substiuted for prety 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.
This fly reminded me of a Robber fly but I am not sure and if it is one, it will be the first of it's kind I have seen locally
On Encelia californica.
30 Apr 2013.
Bowman's Hill Wildflower Preserve, Bucks Co, PA.
Found on a stone wall on a bridge over a wooded creek.
ID courtesy of John Ascher:
~8mm total length. Found this fly on Argyranthemum frutescens in our garden. (live oak/chaparral habitat).
~5mm total length. Saw these two flying about slowly in our yard in the mid afternoon & tried to photograph them in the air, but couldn't get them in focus. They finally landed on a Ceanothus bush. Tentative ID by Isaac Winkler @ BugGuide.
Dagger fly male resting on a dew covered daisy in a wet meadow. Beynat, Limousin, France.
Empis marqueté mâle posé sur une marguerite couverte de rosée dans une prairie humide. Beynat, Limousin, France.
Dagger fly female gathering nectar of a wood scabious flower covered with dew in a wet meadow. Beynat, Limousin, France.
Empis marqueté femelle butinant une fleur de knautie des bois couverte de rosée dans une prairie humide. Beynat, Limousin, France.
Empis sp. cf. Empis spectabilis (Diptera: Orthorrhapha: Empidoidea: Empididae: Empidinae).
Occoquan Regional Park, Fairfax County, VA 28 April 2013
DIP Empidae Empis