Carpelimus erichsoni

I caught these three ostensibly identical rove beetles at light. One managed to escape after I'd taken its photo but the other two were keyed using Lott. They keyed fairly straightforwardly to the species pair bilineatus/erichsoni, but using Lott, these two species are only differentiated by reference to the male genitalia. One was a female but fortunately the other was a male. However, this did not mean that all was plain-sailing! The long callipers immediately confirmed that I hadn't gone wrong up to here - they were indeed one of these two species. The callipers seemed dark and broadened similarly to how they're illustrated for bilineatus. This wasn't immediately obvious as the very dark part remains narrow, the broadening being paler and less obvious. The shape of the rosette, however, did not precisely match the diagram for either species, though I considered it was much closer to bilineatus.

Looking further into it I came across a 2021 article by Sapeijai and Melke describing the first record of erichsoni for Poland, which contained photos of both species' genitalia. Confusingly the rosette looked different on both, and the callipers weren't such a great match either. Then I found the Käfer Europas website key which further confused me at first. This shows 3 diagrams and 2 photos of bilineatus genitalia (including some of what it calls "the form described as augustae" (as translated by Google), which it says is the same species as bilineatus. The callipers looked ok, but the rosette didn't look exactly like mine - but there was quite a bit of variation between the pictures so I think this must be quite variable (or can vary according to position etc.). It only has a single line drawing for erichsoni, but this also looks unlike my specimen in both callipers and rosette.

In the end I decided that since the callipers best matched bilineatus and not erichsoni as shown and described in Lott and at Käfer Europas, and identified my specimens as bilineatus. The beetle was about 2.6mm long when examined but may have contracted after death.

Then in 2026 I caught further Carpelimus specimens and this time the relevant volume by Duff had been published. To separate bilineatus from erichsoni, Duff actually describes the differences in genitalia, ignoring the rosettes that I'd been so confused about and focusing on the colour and thickness of the calipers. Now with this in mind and comparing my photos with the photos in Sapeijai and Melke, I realise that mine 2023 males were in fact erichsoni. The calipers are solidly dark in bilineatus, whereas Sapeijai and Melke's erichsoni they are unevenly dark, dark along the inner edge but overall paler. Although broader in the basal half than perhaps suggested/shown by Duff, the calipers clearly narrow towards the tips which they hardly do on bilineatus.

male Carpelimus erichsoni showing head, antenna, pronotum and elytra (4 views) and aedeagus (3 views), Wendling Beck Project (Norfolk, UK), 6th September 2023

As this arrived more-or-less with the male above it's probably safe to assume it's the same species. Käfer Europas shows a spermatheca for bilineatus but not for erichsoni, and differences between the spermatheca of the two species, if there are any, are not described. The beetle was 2.5mm long when examined but may have contracted after death.

Duff illustrates differences in female ring sclerites, but unfortunately I did not know to look for these when I examined these in 2023.

female presumed Carpelimus erichsoni showing head, antenna, pronotum and elytra (4 views) and spermatheca, Wendling Beck Project (Norfolk, UK), 6th September 2023

The third one that escaped appeared to be the same, and seems most likely to be the same, but I suppose I cannot exclude the possibility that it was a different species, potentially including a completely different Carpelimus species.

presumed Carpelimus erichsoni, Wendling Beck Project (Norfolk, UK), 6th September 2023

Two nights later at a different site within the same project area I caught two more bilineatus/erichsoni, a male and a female. As before, I identified it at the time as bilineatus but now consider it to be erichsoni.

male Carpelimus erichsoni showing pronotum and genitalia (2 views from opposite sides), Wendling Beck Project (Norfolk, UK), 8th September 2023

This is the female caught with the male above.

female presumed Carpelimus erichsoni showing antennae, head, pronotum, head to elytra and spermatheca, Wendling Beck Project (Norfolk, UK), 8th September 2023