Acanthocnema nigrimana
A brownish fly with yellow legs and the front of the frons yellow. The third antennal joint is also yellow with a variable amount of darkening at the tip (darker in females according to Collin, 1958)[1]. Wing length: ♀ 4.6 - 4.6 - 4.7 mm (2).
References
This appears to be a scarce species of western Britain where it is found beside streams and rivers on vegetated shingle banks, usually in wooded valleys. Larvae unknown, but presumed to be similar in habits to A. glaucescens. Regarded as a rare species, but may be overlooked (see discussion under the next species).
World distribution: Palaearctic: Austria, Denmark, Germany, Hungary, Norway, Poland, Portugal, Sweden. British distribution: Recorded from Devon, Somerset, Yorks, Durham, Brecon, Caernarfon, Stirling, Inverness and the Isle of Lewis.