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Fly from 2 april 2017 riverine area Nederrijn river central Netherlands How to distinguish S. micans from aurata What do we see on the pictures of the genitalia
Any help with this puzzle appreciated

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pict nr 2

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pict.nr 3

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pict.nr.4

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pict.nr.5

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I don't think this is either micans or aurata. Unfortunately it's very tricky to get sufficiently detailed genitalia pics, but this looks like either trina or breviventris. You can download a copy of Mathis' 1982 Revision of Setacera from https://repositor...10088/5662. If you're still in doubt, I'd be happy to borrow the specimen and name it for you. Another new one for the checklist!

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Tony, thank you for the Setacera key. I have tried to use it and preliminary arrive at S. breviventris.
see 3th and 4th tergite about same size and each about 1,5 x larger than 5th tergite

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posthumeral weak, that is weaker than largest humeral bristle

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surstyli acutely corniform apically . Here my knowledge/ experience is meagre Probably more like guesswork. Which one is the surstyli??

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The surstyli are the projections at the tip of the epandrium (the external genital segment that you have tweaked downwards in these specimens). In some ephydrids the surstyli articulate with the epandrium. In some (like Setacera), the surstyli are fused with the epandrium and each other. In your final picture and in the first, you can see the small pointed projection at the tip of the surstylus, which would agree with breviventris. However in your 6th picture, tergite 5 appears to be about the same length as tergite 4. Of course, it's not unusual for two or more species of Setacera to occur together. Unfortunately I have no male trina to hand, so I cannot eliminate that species. Best to get them checked before adding to the list! :)

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The 6th picture of the Setacera with untreated genitalia is of the same specimen before treatment. The band you see on the tergites is the distal part of each tergite. I have measured the tergites by enlarged photo of the profile. The 5th tergite is clearly much shorter than the 3th and 4th.

Perhaps at first glance the difference looks less obvious