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7mm Clitheroe England 14 May 2010.

I can't key this out to anything sensible. Can you help?

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Please, to begin, can you check for presence of setulae on posterior/inner edge of hind coxa (that would suggest Thricops). It is an Azeliini (strong proclinate orbitale).

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As far as I can see there are no hairs or bristles there. Plenty of strong bristles on the side facing you in a dorsal view of the fly, but as you look further round behind the coxa the surface is smooth and rounded. I'll try to get a useful photo, but it's difficult to get in there - the coxae are short and close together. Without dismembering the specimen I'm not sure what happens where they touch.

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Here is a lateral view of the hind coxa

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and this is the hind coxae taken from the front of the fly. I may be confused about which is the posterior inner margin :|

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OK, we have ruled out Thricops (hind margin of cx3 bare). It is either Hydrotaea or Potamia. I think the second, because the genae are rather high, and the subcosta is bent in the middle. So you have to check these points to confirm my guess : arista is plumose ? And katepisternum with 1:2 setae (1 in front, 2 behind, the lower posterior shorter than the upper, but conspicuous). Tell me...

For your information, I attach your picture, annoted so that you can see where the setae are to be found on the hind coxa for genera Thricops (and Azelia).

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Many thanks Stephane. Actually Potamia was where I kept ending up in the key, but I misread the distribution tables and thought we didn't have it in Britain:@ P. littoralis fits OK. The arista is distinctly plumose but the hairs are not very long, and the katepisternum is exactly as you describe.