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I looked at Seioptera vibrans again. In the Seioptera vibrans pictures in the gallery veins R₄₊₅ and M₁ converge to the wing tip. In my pictures, the veins R₄₊₅ and M₁ are almost parallel at the tip. So I suspect it might be a different species.
The tribe Seiopterini includes in Europe Seioptera vibrans, Pseudoseioptera demonstrans and some species of the genus Homalocephala (Fauna Europaea). Together with Seioptera vibrans Homalocephala albitarsis, H. biumbrata and H. mamaevi are recorded in Germany.
In the key of UK species is mentioned that larvae of Homalocephala biumbrata develop under the bark of fallen aspen.
The 2 UK species differ in the color of legs (H. albitarsis: completely reddish and H. biumbrata: dark-brown to black) and in the restriction of brown coloration to cell Sc (H. albitarsis) or further extension to r-m (H. biumbrata).
Could the flies shown here belong to the genus Homalocephala, perhaps H. biumbrata?
If so, the yellow halteres would speek for biumbrata and exclude H. mamaevi with dark brown halteres.(Homalocephala key of E. P. Kameneva & V. A. Korneyev)
But H. biumbrata I could neither find in the Catalogue of Life nor in Systema Dipterorum. A search for the basiomym Psairoptera biumbrata Wahlberg, 1839 in Systema Dipterorum resulted in the valid name Homalocephala albitarsis Zetterstedt.
Now I am puzzled.

Marion