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This fly looks like no other Chrysogastrini I have seen before. I have no idea what it can be! It appears to belong to the genus Melanogaster, as it has a dull sternite 1 (with a tiny hint of shine on the lateral part of the first tergite), black third antenall segment, and all black legs. However it has a very unusual face profile with no knob - see photo. I took a pair in copulation and the female has the face profile of an Orthonevra species, with no knob and concave in profile. However, the female also has a dull first sternite, and black antenna. Neither male or female has any evidence of stripes on the dorsal surface of the thorax.

I thought this might be a mute or aberration, but I have another male and another female, both with the same characteristics. The overall appearance is of a small black fly about 7mm in length (antenna to tip of abdomen). I took all four specimens in a marsh on Caltha palustris flowers on 2 May. Shropshire, United Kingdom.

Photo is of male. Note the very angular profile. Can anyone suggest what species or genus even, this might be?