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Thank you for your help, Robert and Menno.
The first few Tarsi should be yellow for C. soror, shouldn't they?
When I follow the key from Bothe (which does not include all Cheilosia, I know, but which key does?), I end up at " Cheilosia cynocephala, mutabilis, proxima, praecox, vernalis, velutina".
Some of those should be exkluded, because the fly has bare eyes, and on the other hand I am still not sure if those are all species, I should put in consideration.