Compost Fly

Ptecticus trivittatus
Ptecticus trivittatus

I captured this fly after dark, at our front porch light in early August, 2013. After capture, the fly spent the night in a vial in our refrigerator. The following morning as soon as there was plenty of light, I released it. Being chilled, it stuck around long enough for a few photographs.

Looking at this fly, and the photographs of the fly, I was immediately struck by it’s colors, the translucent greens and ambers, its size and shape, the venation and rippling look of flowing water in the wings. Only later could I identify it as a Yellow Soldier Fly, Ptecticus trivittatus. This fly, common in eastern North America, is most often observed near compost piles and fermenting fruit, so why was it hanging around the porch light after dark? Those big compound eyes and the pronounced ocelli indicate it’s well adapted to low light conditions; maybe it’s just that the earthy wine these flies seek, decanted from decaying lawn clippings and fallen fruit, is better in the shade. My subsequent encounters with this species placed it at our compost pile, but it also showed up again after dark, at the mothing light on several occasions.

Ptecticus trivittatus
Ptecticus trivittatus

Looking for more information about the fly on the web, I came across a short note in the journal Psyche by Philip Rau, author (along with his wife Nellie) of Wasp Studies Afield (1918). Here are a few passages in summation of our compost fly, in Rau’s characteristic flouncy style, written around 1930, in or near Kirkwood, Missouri:

“…these attractive greenish-colored flies were seen hovering in courtship dance above garbage heaps on the rear of a lot… To say that they dance incessantly is not wholly true for individuals often leave the throng to rest on a tin-can or bottle or cantaloupe skin… indeed, it’s a pretty sight to see a flock of these flies moving in a horizontal plane in more-or-less irregular circles and in figure eights just an inch or two above the mass of multi-colored refuse.”

Ptecticus trivittatus (total length=14mm)
Ptecticus trivittatus (total length=14mm)

Resources

Bugguide page for Ptecticus trivittatus.

Eric R. Eaton. Kaufman Field Guide to Insects of North America. Houghton Mifflin Harcourt. 2007.

Phil Rau. Notes on the Courtship and Mating of the Fly, Ptecticus trivittatus Say. Psyche 44:141-142, 1937.

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