Cercyon bifenestratus

This came to light alongisde a Cercyon marinus, providing an excellent opportunity to compare the two species. It was larger than the marinus but both were comfortably within the size range for both species (the bifenestratus was a female and the marinus a male which may have been relevant to size?). The mesosternal field was clearly broader than on the marinus and the extent of the orange tips of the elytra were different: the orange extended further back from the apex centrally but did not extend so far along the outer edge towards the base. The dark along the suture was much more obvious, more thickly and obviously dividing the orange tips to the elytra - this is illustrated in Duff but not alluded to in the text, but it is referred to in Hackston. According to Hackston and the Beetles of Europe website (coleonet.de) the second segment of the maxillary palps is thicker on bifenestratus. On my specimens it was perhaps slightly thicker, and certainly a different shape being more evenly roundedly thickened in the middle compared to the marinus which was thickest nearer the apex.

female Cercyon bifenestratus showing mesosternal field, metasternum, elytron and maxillary palp, Wendling Beck Environment Project (Norfolk, UK), 29th June 2025

On the above beetle the mesosternal field was obviously different from the marinus I examined with it, but I didn't think it was quite as broad as illustrated in Duff. However it compares well with photos of bifenestratus online that apppear to be reliably identified, so I think this is fine. One of the beetles I had previously logged as marinus had been challenging as the orange did not extent back along the edge of the elytra in the way that it should in marinus. At the time I compared the mesosternal field with the diagrams in Duff and I didn't think it was broad enough for bifenestratus and concluded that it must be an unusual marinus, but in view of the subsequent one not being quite as broad as shown there, I thought it was worth looking at again.

Compared to others that I've identified as marinus this one (and one other) do in fact have broader mesosternal fields, and furthermore the orange is more extensive centrally and the black divides the orange patches more broadly than on my other marinus. What's more I had also been slightly bothered by the fact that the aedeagi did not perfectly match the aedeagus of marinus shown at coleonet.de. I had convinced myself this wasn't too important after finding another photo of a purported marinus aedeagus online that was similar. The aedeagus of the marinus that I examined with the above bifenestratus matched the marinus at coleonet.de perfectly, with a more pointed tip to the aedeagus than the previous, which added to my conviction that the other was really bifenestratus not marinus. I should add here that there is not a description or photo of bifenestratus aedeagus at coleonet.de, nor have I found these anywhere else, so this is really a steer away from marinus rather than necessarily a steer towards bifenestratus.

Note that the central portion of the aedeagus is loose and can slide up and down in relation to the outer parts. As I cleaned it I accidentally pulled the central portion out - 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 bifenestratus 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

Like the last one, I had originally, but uncomfortably, identified this as marinus. The orange at the sides of the elytra extends a bit further towards the base but not as far as it typically does on marinus, and the other elements of the pattern match bifenestratus better.

male Cercyon marinus showing side profile, fore tibia, mesosternal field, elytron, elytral epipleuron and aedeagus (side and flat views plus close-up of tip), Wendling Beck Environment Project (Norfolk, UK), 25th June 2025