Peritrechus geniculatus

This was found by suction on an ex-arable lowland meadows. Separation from Peritrechus nubilus seems to revolve around the thickness of the third antennal segment and the shape of the pronotum. The third segment should be as thick or thicker than the first segment on this species, thinner on nubilus. In fact I made it very slightly thinner on this one (0.12mm vs. 0.13mm) but I think these are close enough to be considered about the same. Comparing with photos of both species (including my own of nubilus) it seems to fit geniculatus best on both antennal width and pronotum shape. Both species are supposed to be common.

Peritrechus geniculatus showing front femoral spines, Wendling Beck Environment Project (Norfolk, UK), 25th August 2025

This was found by suction in exactly the same place. Again the third segment of the antenna was 0.11mm vs. 0.13mm for the first, so strictly speaking this keys to nubilus, but I don't think that's correct. So far as I can tell from photos, most bugs identified as geniculatus on the internet (including at reliable sites like British Bugs) look like they have the maximum width of A1 marginally wider than the maximum width of A3. Looking at photos of both species A3 is more obviously narrower than A1 in nubilus, and also narrower than A4. The Kirby draft key doesn't provide any other criteria but British Bugs alludes to pronotum shape, for which I think mine fits geniculatus best. It also says nubilus usually has a brown anterior margin to the pronotum which both of the ones I've identified as geniculatus have lacked, and they are also both darker than the one I identified as nubilus.

Peritrechus geniculatus showing front of pronotum and fron femoral spines, Wendling Beck Environment Project (Norfolk, UK), 26th September 2025