4mm long, a parasite of Crane-fly larvae
No interactions present.
the median marginal bristles are on abdomen 1+2. This is another fly with jointed aristae
latest pics and diptera videos
The arista has a very short 1st segment, a medium-length 2nd segment and a long 3rd one, although these are not keying features.
For another genus with an arista of three segments, see Triarthria:
The T1+2 marginal bristles can't be relied upon - Belshaw was incorrect to say that only geniculata and setosa have them - they are also present in other species, namely cristata (rarely). Siphona geniculata is still the most likely identification (going by the large number of bristles below the last parafrontal bristle) but I couldn't be 100% sure unless I had seen the specimen.
Lat/Lng: 51.5557, -0.0246
OS grid ref: TQ370859