Stenus picipes

I used Mike Hackston's keys to identify this beetle which was swept from some grass or a sedge in alder carr woodland close to the edge of a lake. It seems to key to picipes relatively straightforwardly. The only discrepancy was the size, which I measured at 3.5mm long, half a cm less than the minimum size given by Hackston. However, I measured the dried specimen and comparing it to the live photos it was clear that the abdominal segments had withdrawn into one another significantly reducing its overall length. I would have liked to have been able to double check by comparing its male genitalia to the diagram in Hackston's key, but on dissection it proved to be a female.

Stenus picipes showing antenna, hind tibia and tarsus, other hind tarsus and close ups of head + pronotum and basal abdominal segments, Wendling Beck Environment Project (Norfolk, UK), 26th May 2023

This copulating pair was found by suction sampling at the same place as the one above. They keyed to picipes using both Lott & Anderson and Duff, but even so it was reassuring to be able to check the male's genitalia (particularly as at some angles, see last photo below, I thought I might have been able to make out an indistinct central keel on the basal tergites, though after looking from every angle I was pretty sure there wasn't really one).

male & female Stenus picipes showing male's basal tergites and aedeagus (including close-up of tip) and female's hind tarsus + tibia, hind tarsal tarsus closer, foreparts, palp, elytron and basal tergites, Wendling Beck Environment Project (Norfolk, UK), 12th March 2025