Ilybius quadriguttatus

I initially used Mike Hackston's key to identify this. The elytral microsculpture was only obviously elongated close to the suture in the basal region but the other characters checked out fairly straightforwardly. I double-checked with Duff, not least because I wasn't comfortable with size being the only character used to eliminate ater - but this was pretty much the case in Duff too (he also separates them on how matt or shining the upperside is but I find that quite subjective in the absence of reference material to compare with). This one was 11.5mm long.

Ilybius quadriguttatus showing keel on last segment of abdomen, elytral microsculpture (near middle of elyra and base around suture) and outer front claw, Wendling Beck Environment Project (Norfolk, UK), 11th June 2023

Given my concern mentioned above in relation to eliminating Ilybius ater purely on size, a learning experience in 2025 is worth noting here. I caught a pair of Ilybius ater, the female of which was within the expected size range but the male was slightly under. I was confident both were ater despite the sizes as the elytra were identically matt, but followed the key through for the smaller one as if it wasn't. It reached a dead-end at Ilybius quadriguttatus because although size suggested that species, the front tarsal claws did not have a ventral tooth like that of quadriguttatus.