Not far from the dead squirrel, I encountered something else that was attracting a lot of flies. The smell was far more intense and unpleasant than from the squirrel parts, yet it wasn’t decomposing at all, but in the prime of its life: a pair of Ravenel’s stinkhorns (Phallaceae: Phallus ravenelii). Stinkhorns are mushrooms with an unusual spore dispersal strategy. Whereas I suspect that spores of most fungi regularly hitch a ride on insects, they are primarily wind-dispersed. Stinkhorns, however, have a coating of stinky slime, which attracts not just the usual fungus-feeding insects, but also flies that normally are attracted to dead things (several photos on BugGuide.net also show carrion beetles visiting stinkhorns). The flies feed on this sticky, stinky coating, in the process ingesting spores and getting them stuck to their feet. If you look closely at the photo below, you will see several smaller flies in addition to the big one.
Before I even arrived at the stinkhorns, a tiny (2 mm) fly appeared in front of me, hovering like a hover fly (Syrphidae), but it clearly was not one. There were several of these same flies hovering a foot or so above the stinkhorns.
I was reminded of this photo of a braconid wasp, which James C. Trager described as exhibiting “phorid-like behavior” by hovering over an ant nest. This is the only reference I can remember to a non-syrphid fly hovering in place like this. When I submitted the photo below (one of the same flies resting on a stinkhorn cap) to BugGuide, John F. Carr assured me that it was not a phorid. Then, in searching “stinkhorn” on BugGuide I found some photos of something that looks very similar, with this comment left by Jeff Skevington: “These flies are platypezids [flat-footed flies]. . . I believe that they are Melanderomyia kahli Kessel (or an undescribed species of Melanderomyia). These are rarely collected or observed flies that specialize on stinkhorns.” Scuttle flies (Phoridae) and flat-footed flies belong to the same superfamily (Platypezoidea), so maybe I wasn’t so far off… except both John and Jeff think mine are some kind of acalyptrate fly, of which there are many to choose from. So I have no idea what they are. [10/14/2012 edit: dipterist Terry Wheeler, however, agrees with me that they look like Melanderomyia.]
The big fly in the top photo is a blow fly (Calliphoridae), belonging to the same family as the dominant flies on the squirrel remains. Ben Coulter has suggested that the species is Cynomya cadaverina.
Most of the smaller ones were vinegar flies (Drosophilidae), just like the ones that frequent fermenting boletes.
The other flies I saw visiting these stinkhorns belonged to the family Heleomyzidae. There were individuals of two different species of Suillia feeding on the caps, and both exhibited that charming bubble-blowing behavior (which I first saw in association with a bird dropping) when they had had their fill.
“Cotinis,” who took most of the photos that pop up when you do a “stinkhorn” search of BugGuide, posted photographs of a Suillia male exhibiting territorial behavior atop this same kind of stinkhorn, “often holding one wing out in an obvious display.” The only heleomyzid I had photographed previously was doing a similar display, but on a leaf. According to Cotinis’s comments, Suillia larvae feed on fungi, so these flies may have been here to do more than just slurp the slime.
Great post and spectacular pictures. I wonder if your mystery fly is a a frit fly in the Oscinellinae? Looks a bit like one but, as you say, there are a bewildering diversity of acalypterate flies. When I want to recalibrate my key writing abilities, I try to key a couple of mystery acalytperates in Nearctic Diptera, note all the frustrating bits, and think about how to make my keys avoid similar problems.
I’m not sure where you are at, but Melanderomyia species are known from Kansas to Pennsylvania according to Nearctic Diptera and the BugGuide pictures are from North Carolina, so sounds like they may not get very far north. The generic key to the genus is entirely based wing vein characters (dm-cu absent, r-m present, M1+2 not branched), but looking at your images, I certainly could not rule the genus out.
Thanks for explaining the hypothesis about bubble-blowing and evaporation. Seems reasonable. I have a picture of a frit fly doing the same and wondered if it was a form of chemical signalling.
Thanks for the ID suggestion–it certainly does resemble an oscinelline, at least superficially. I’ll have to collect a couple of these flies if I see them again. I found another stinkhorn today but they were not in evidence.
I’m in western Massachusetts, so that’s one more strike against the possibility of these being Melanderomyia.