39. Psilidae – character

Have you ever seen such a boring fly? It’s a pallid colour with see-through legs, hardly a bristle on it apart from the remnants of a bad hair cut on the back of the head, and an undistinguished pattern of veins on a not-quite clear wing. There are no unusual thickenings of the legs or distortions of the thorax, the antennae are just the normal three segments with a slightly feathery arista (the thin hair that sprouts from the base of the third antennal segment) and the abdomen is just a uniform blob of bland yellow brown. Even the common name of the family – rust flies – derives from the rusting produced by the larvae of Psila rosae, the carrot fly, on carrots, rather than from anything interesting about the flies themselves.

Yet it turns out, that in the same way that people are always more interesting than mutual reticence might at first suggest – like the stranger you get talking to at the bus stop (now my dream analyst, squash pupil, bicycle mechanic and cake-sharing buddie), the viola player in your amateur orchestra (despite all the viola jokes, now my wife), or the furtive net-swisher in the woods (actually, its probably best to keep well away from them), even this fly has character when you spend a little time getting to know it better.

For a start, the head is an unusual shape, receding severely from the a pointed front, the antennae slung below, and no hint of an upper lip or moustache. Another peculiarity is that about a third of the way along the front of the wing there is a faint vertical line where the membrane is clearer than elsewhere, and this clear line wanders down the wing, slanted back towards the body, like a tiny snail trail. That subtle feature and the absence of bristles in several crucial places are the characters that place this fly in the family Psilidae.

Following the key by Darwyn Sumner, and eventually realising that I had put the pin through the exact spot on the thorax where a crucial bristle should have been (visible however, on the other side) brought me to the conclusion that this was Psila fimetaria, a widespread species of low plants in damp places, which is indeed where it was found. The final couplet asked if the antennae has a dark patch where the arista emerges from the antennae, and when you get up close, it turns out that it does. Had we not got to know each other so well, I would never have noticed that affecting detail. None of us are entirely boring.

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