Cercyon marinus

Using the Duff key I went a bit wrong with this one at the sg. Cercyon key, couplet 4, where I had to decide if it had femoral lines or not. I wasn't quite sure what femoral lines were but there was a handy diagram showing the femoral lines of impressus, but there is a limit to how much of a 3-dimensional structure you can get across in a 2D line drawing. My specimen had a ridge running along the line precisely where the femoral lines are indicated on the diagram, so I figured that was what it meant and pursued the option for having femoral lines. After getting in more of a muddle and reaching no clear conclusion that way I turned to Hackston's key, and this time went straight to marinus. Once I clocked where I went wrong in Duff, the rest of that key did also lead to marinus.

Cercyon marinus showing evenly-curved side-profile, mesosternal field and metasternum, Yarrow Bridge, North Elmham (Norfolk, UK), 23rd March 2022

This one keyed to marinus using Duff fairly smoothly. The yellow on the elytra appeared to stop well short of the humeri looking from above but when viewed from the side you could see it extending further forwards. Checking against Hackston I hit a dead-end - the problem seemed to result from this key requiring the front angles (at least) of the pronotum to have a reddish patch whereas on mine the proonotum was completely black. My photo of the hind angles of the pronotum does appear to show a slight reddish reflection at the front margin of the pronotum near the front angle, but even after noticing this in the photo I could not find any angle to hold the beetle at where I could see this under the microscope. My previous example above did have such a reddish spot and so do most examples I can find online, but the extent seems to be variable and as Duff doesn't mention it I'm hoping it can be missing altogether. This beetle was 2.6mm long.

Cercyon marinus showing pronotal hind angle, mesosternal field, metasternum, elytra from side and comparison of elytra from above with Cercyon analis (marinus above, analis below), Wendling Beck Environment Project (Norfolk, UK), 14th July 2024

The next two were caught on the same night as the last, but in a slightly different place. They came to light. The first one keyed to marinus fairly straightforwardly. It was 2.7mm long.

Cercyon marinus showing hind tarsus, head, mesosternum and elytron, Wendling Beck Environment Project (Norfolk, UK), 14th July 2024

A cursory look at the second suggested it might be something different. Not only was it smaller (2.4mm) but it had yellow elytral epipleura (or more precisely, about half the width of each epipleuron was yellow). Comparing the two side by side the elytral striae were slightly less prominent on this one, including the sutural striae. But taking it carefully through Duff and Hackston I still ended up at marinus. A search of images of marinus online turned up examples both with black or with yellow epipleura, so presumably this is within normal variation (sex-related maybe?).

Cercyon marinus showing hind tarsus, head from side and front, upperside, close-up of elytra and mesosternum, Wendling Beck Environment Project (Norfolk, UK), 14th July 2024

In most respects this one looked like previous marinus that I'd encountered, but the yellow-orange on the apex of the elytra did not extend so far up the sides of the elytra. This caused me to seriously question whether it could be marinus, but checking and rechecking all the steps in the key I couldn't make it anything else. The elytral epipleura were yellowish to the base but the yellow on sides of the elytra didn't extend at all far up. I resorted to dissecting it and the aedeagus appears to confirm it is marinus. Neither Duff nor Hackston provide details of genitalia but the Beetles of Europe website provides illustrations and/or photos of the aedeagus for most species. As one of the differences between the various species seems to be the position of the central portion relative to the sides (parameres?) it's worth noting that the central portion is loose and can slide up and down. As I observed the aedeagus prior to cleaning it up for the photo it matched the photo for marinus, but when I cleaned it I pulled the central portion out accidentally - I managed to push it back in but not quite to the extent it had started off, so bear this in mind when viewing the photos of the aedeagus below.

male Cercyon marinus showing side profile (2 photos), side of elytron including epipleuron, mesosternal field, clypeus and aedeagus (with close-up of tip), Wendling Beck Environment Project (Norfolk, UK), 21st September 2024