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the only way to go with female Bibio, in my opinion, is Verbeke (1971) [ Bull. inst. r. Sci. nat Belg. 47-23.
Yes, female varipes have red venter, but so do many lanigerus.
The hind metatarsus is much shorter in johannis than in your picture, so it is pretty safe to say this is not johannis. As you mentioned, johannis has dark venter.
There are several very subtle features separating varipes from johannis, but based on these pictures, I would not dare to make the call. The reddish sport on pleura is not very convincing here (much clearer in my material) and the shape of the prothorax suggests lanigerus rather than varipes.

So, I wished I had taken my own advice: don't go here, unless you can associate your female with a male ....


Theo