Sunday, July 11, 2010
The Highs and Lows of Boatman Flies
Here is a boatman fly, pogonortalis doclea, plying the waters between the spikes of artichoke leaves in the backyard. I thought this looked cool, like the fly was on a mountain peak with a hazy high altitude sky in the background. I often see these flies in this kind of setting, on a leaf usually solo, beating the wings in an oar-like fashion. Sometimes I see two of them mating on a leaf but not as much.
Looking around the internets, I find the wing-beating is said to be a male display. Either to attract a female or discourage a competing male, I suppose although the internets did not clarify. Males are also said to engage in head to head combat, again presumably to defend a territory with potential females to mate with. I wonder what else do these flies do with their time; what do they eat, where do they lay their eggs, how do they pupate? Many online sources state that "adults are attracted to mammalian dung to feed". I was tempted to repeat that obvious-seeming morsel of information here, when I realized I was reading the same thing verbatim in various sources, as if in an echo chamber of unverified although plausible data.
So I had a look. Thanks to Gizmo the wonder dog, there was some fresh mammalian dung in the backyard. And sure enough there were several boatman flies on said dung. Two appeared to be males, waving their wings and briefly engaging in what could have been described as head to head combat. Then one of those flew off. Two others were engaged in coition right there on top of the substrate. They remained in the mounted position for some 15 minutes (or more: they were already coupled when I first showed up on the scene). Eventually the male flew off, then the female sniffed around the dung and the ground under the dung. I did not see her lay eggs, although I suspect the ground beneath the dung could be the site of egg laying.
On the dung are marks that appear to be pits from feeding activity. I did not actually see these flies feeding, however, as they were otherwise occupied at the time. Plenty of flies are attracted to dung, including this green bottle fly (lucilia sericata) that seems to have a prurient interest in the goings on on the dung. A flesh fly also popped in for a look, and scared off the second solo male boatman fly.
It seems easy to assume that boatman flies not only are attracted to mammalian dung to feed but actually do feed on it but is it safe to say that? I do know they hang out on dung, mate on dung, scrounge around on dung and wave their wings on dung, as well as doing all those things on nice, clean artichoke leaves.
Last, can I mention how unpleasant dog poop is to look at in photographs, as it seems to readily trigger an involuntary disgust response even though the photo poses no actual unpleasantness to the viewer?