Cercyon lateralis

I initially used the Hackston key to identify this, and identified it as lateralis but I had a couple of concerns with this ID. Firstly it was a quarter of a mm longer than the top end of the size given there (3.2 mm - Hackston gives it as 2.5-3.0mm) and secondly I wasn't convinced about the border along the edge of the pronotum stopping at the hind angles (although it was so indistinct along the base I wasn't sure it counted and I could only see it at all on one side).

So I checked the ID using Duff (where the size range goes up to 3.2mm). Here I had to decide whether the yellow apical mark on the elytra was sharply-defined or not. On the specimen under the microscope it didn't look sharply-defined (or particularly obvious at all when viewed with a pale background) but in my photos of the live insect it did look quite sharply-defined. If I took it not to be then Duff also ended up at lateralis, but if I took it to be sharply-defined then Duff took me to marinus. But there were problems calling it marinus, because the mesosternal field shouldn't be as narrow as it was on my specimen (it's so narrow I'd call it a keel) and the palps should be darkened. I checked back with Hackston to see where marinus diverged from lateralis in his key, and it was the darkness of the palps - at least "distinctly darkened" for marinus etc. and "yellow to yellowish-brown, sometimes indistinctly darkened or with darkening restricted to the last segment only." for lateralis etc. You could describe them as slightly darkened, espeically the last segment, which fitted lateralis fine, but it would be a stretch to call them distinctly darkened.

As a final check I looked up some images of both species, including the underparts (including my own for marinus), and having compared my specimen with these I was happy that marinus was ruled out and lateralis seemed good.

When genitalia aren't shown in the keys I'm never sure if this is because they aren't helpful for the ID or just that the authors deem that the species are easy enough to identify without referring to the genitalia. I figured I might as well photograph the aedeagus just in case it might help resolve a future challenge.

Cercyon lateralis showing evenly-curved side-profile, pronotal base/corner, mesosternal field, metasternum, elytron and aedeagus (3 views), Wendling Beck Environment Project (Norfolk, UK), 21st August 2023