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I could have known this..:@, I spent some time on these flies some days ago..

In early spring this could be B. Johannis.
I see a dark pterostigma, but is it black or brown..?
The light hairs on the thorax makes me think of B. Lanigerus..

On another picture I can see clearly a big spine on the frontal leg, this means that it is a Bibio sp. anyway!

On the frontal leg the metatars 3 = small and long, Bibio varipes could be the specie.
Also the time of flying and the biotope (woodforest) fits..
I believe it is Bibio varipes :)

But the legs of this male are not black?!, which I have seen on other pictures of B. Varipes..

greets, Robert :)