I found these two strange mating flies on bare chalk yesterday. On a closer inspection, these two appeared unusual in appearance. Pity about the blade of grass. I have not got a clue what species or family they belong too.
Just look at the incredible stacked photograph of the Robberfly Machimus atricapillus, the species shown in my original image. Also a box of British Diptera species including several Robberflies from the family Asilidae. Both on display at the Dipterist's Forum at the AES fair.