Monthly Mystery #23: A Frothy Balloon

Although unidentified leaf mines are the main mysteries occupying my mind lately, I don’t want y’all to get burned out on that topic, so I just spent a little time searching through my photo archives for something else to ponder. I came across this 4-mm object I had photographed on July 1, 2008, somewhere near Asheville, North Carolina.

My impression at the time had been that it looked something like a green lacewing cocoon. But besides being a little too large (green lacewing cocoons seem consistently to be 3 mm long), magnification reveals that this object is composed of some kind of hardened frothy material, rather than silk, as is the case with this green lacewing cocoon I found on a nearby leaf:

My current suspicion is that the mystery object is a nuptial balloon dropped by a dance fly (Empididae) in the genus Empis, subgenus Enoplempis. These are fascinating flies that I hope to get to see in action someday. Males of some species offer prey items to females and can be seen carrying them in flight, as shown here. Males of other species package their gift in a frothy balloon, as seen here. Others, such as E. snoddyi, go a step further and offer females empty frothy balloons like this one. Apparently the timing of my July 1 discovery is reasonable, since this E. snoddyi was photographed carrying a balloon in North Carolina on June 25. According to BugGuide, E. snoddyi is the only balloon fly in the southern Appalachians, so if I’m right in supposing this object was made by a dance fly, it apparently can be attributed to that species. Anyone have any other ideas?

Based on the current BugGuide map, Enoplempis is not found in the northeastern US, so I guess I will have to head south (or west) some spring if I want to see these flies. However, members of a related genus, Hilara, make similar nuptial balloons using silk rather than froth. I have seen a single Hilara individual not far from where I live in western Massachusetts:

…so maybe there’s hope that I’ll get to see them carrying their silk balloons around here someday. As you can see, Hilara males have big Popeye-like forearms. When I started writing this post, BugGuide had no photos of males with balloons identified as Hilara, but I just noticed that the ones in this swarm have the same swollen fore tarsi.