Left: Male, Cambridge 13 Apr 2002.
The males are smaller than in B. marci (below), and the females are distinguished by the red-brown thorax.
Bibio marci (St Mark's fly)|
Male, Cambridge 5 May 2001.
The larger male - the females are all black, and appear on or about St Mark's day, (25th April) hence both the popular and scientific name of the species.