Pericoma nubila (or Pneumia nubila)

Knowing how hard this family is to identify to species level I rarely used to bother trying, but on noticing a pair of flies copulating (or at least back to back as if they were) on a hazel leaf I figured one would be a male so at least I wouldn't have the problem of finding I'd only retained a female (females don't seem to be covered in the key). It would be worth a try.

Several of the features needed (eye-facets, frons, palps, early antennal segments) are on the face which is quite hard to see as there are long tufts of hairs, but I managed to see most of the necessary characters after clearing it in KOH and gently removing remaining hairs. Genitalic characters also helped rule out a number of species when I wasn't quite sure which way to go in the key. Two species (nubila and trivialis) have similar genitalia so in the end I relied on the long frons and the swollen second segment of the palps (although it wasn't as swollen as one photo I found online) to eliminate trivialis. The fore tibia should also have helped but unfortunately I managed to lose both of these while I mishandled the fly attempting to see some of the other characters.

Pericoma nubila, Dillington Carr (Norfolk, UK), 13th November 2021 (I'm not quite sure if this was the male that was identified and features below, or the female that it appeared to be copulating with)

male Pericoma nubila parts (forewing, cercopod with 4 retinacula, gonostyles and aedeagus, face including eye facets, frons and one palp), Dillington Carr (Norfolk, UK), 13th November 2021

A group of half a dozen similar moth flies on a birch trunk consisted of two size classes, so I retained one of each in the hope that one would be a male. The smaller of the two was a male and although I won't say it was easy, it keyed straight to nubila. As before, some characters were hard or impossible to see beneath the fly's hairs so needed to be cleared (I did it in potassium hydroxide) first.

male Pericoma nubila showing wing (toplit and backlit), Dillington Carr (Norfolk, UK), 28th March 2022

This individual had 5 retinaculae on both sides.

same male Pericoma nubila showing retinacula (2 photos of each side), Dillington Carr (Norfolk, UK), 28th March 2022

The hairs make it hard to see the antennal features (and hard to see which segment is which) but clearing it (second photo below) makes it easier.

male Pericoma nubila showing antenna (before and after clearing), Dillington Carr (Norfolk, UK), 28th March 2022

The aedeagus doesn't fully resolve the ID as Pericoma trivialis apparently looks similar in this respect, but it does give confidence that I haven't gone wrong so far.

male Pericoma nubila showing genitalia, Dillington Carr (Norfolk, UK), 28th March 2022

Couplet 13 offers a choice between the eye bridge (at its widest point) being at least 7 facets wide or only 4 facets wide. This individual's eye bridge was 6 facets wide on one side, but on the other side there is a single facet making it 7 wide at one point.

male Pericoma nubila showing eye bridge, Dillington Carr (Norfolk, UK), 28th March 2022

The frons was so covered in hairs that it was impossible to see if it was extensive enough for nubila without first clearing it. This also made it easier to see the swollen second segment of the palps.

male Pericoma nubila showing frons and palp, Dillington Carr (Norfolk, UK), 28th March 2022

This was the larger individual, a female, which I presume to be the same species though this could not be confirmed.

female presumed Pericoma nubila showing wing, Dillington Carr (Norfolk, UK), 28th March 2022