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I don't think this is B. marci, I'll let Ian put us right on this one.
Chris Brooks - www.dragonfly-images.co.uk
My Flickr site - www.flickr.com/photos/ceb1298
This is one of the smaller Bibio - rather tricky to do from Photos - a real need for a microscope - at least for me!
Yes...you need to see the number of antennal segments, colour of body hairs, shape of tibial spurs...etc., to get closer to a species. None can be judged here.
They are mostly in incredibly low numbers this spring where I am...hardly any seen at all, so far, when normally they are just about the commonest easily found family of flies in late April, early May.
The only exception to that is B. johannis, which seems to be commoner than usual, here(but maybe I'm just finding it more easily as there are less of the others around?).
My Flickr photos...
Lat/Lng: 50.7174, -1.8699
OS grid ref: SZ092908