Acrotona muscorum

I was routinely ignoring rove beetles when I found this (in a pheremone lure trap intended to draw in Red-belted Clearwings) but decided, quite randomly, to give it a try using Mike Hackston's keys. With very little experience keying rove beetles I reached a species that I was pretty sure was incorrect - and indeed it wasn't even in the right subfamily. Leaving it aside until I had better experience/references I re-tackled it after acquiring the Lott/Lott & Anderson books. This time I got it to the right subfamily, but Aleocharinae wasn't covered by the books I had so I pended it until Duff was published. So far I haven't had much joy keying Aleocharinae using Duff but he does have diagrams of genitalia so I resorted to looking through the book for a match with the genitalia. Eventually I got it down to a choice between Acrotona benicki (which would be very unlikely I think) or Acrotona muscorum. The sides of the basal section seemed slightly angulate which matched muscorum better but the bottom was perhaps slightly more evenly rounded more like benicki. In the end the detail of the top in flat view (a roundedly triangular piece overlapping a rounded piece) and the internal sclerites in side view (the long curved pointed one and the blunt rounded-ended one to its right both being narrower) pointed to muscorum. I would have double-checked this against the key but after examining it three times and removing the abdomen it was in a pretty poor state - most of the legs were detached and in pieces and several pieces were missing. To separate these two species according to the key I would have needed to compare the lengths of the hind tarsomeres but as muscorum is a common species and I'm not sure if benicki has even been recorded in Norfolk, I didn't feeel the need to try and work out which (if any) of the pieces of leg that weren't missing might have been a hind tarsus. I'm satisfied that the genitalia are sufficient in this case.

male Acrotona muscorum showing aedeagus (flat with parameres and median lobe only in side and flat views with close-up of latter), North Elmham (Norfolk, UK), 22nd April 2019