Carpelimus bilineatus

I'm not sure what the status of this species is - there were only three records of bilineatus and four of the similar Carpelimus erichsoni in Norfolk to 2021, but I think the low number of records may be just a consequence of a relatively recent split. The two species were previously considered to be one and the same so I'm guessing that old records cannot now be ascribed to either species. Lott says bilineatus is "apparently scarcer than" erichsoni, but that one or other is frequently recorded at light.

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 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 were 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, however this was evident enough once cleared properly. 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 helped, though this also 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 all look spot on, 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 matched bilineatus and not erichsoni as shown and described in Lott and at Käfer Europas, my specimen must be bilineatus. Although the rosette wasn't a perfect match, it's closer than erichsoni (no long projections as shown in Lott) and there is clearly some variation in how this can appear. The beetle was about 2.6mm long when examined but may have contracted after death.

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

I guess strictly speaking the female should be left as either/or, but given it arrived more-or-less with the male I think it's probably safe to assume it's another bilineatus. 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.

female presumed Carpelimus bilineatus showing head, antenna, pronotum and elytra (4 views) and spermatheca, Wendling Beck Environment 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 bilineatus, Wendling Beck Environment 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. The male again proved to be bilineatus upon dissection, and I assume the female was bilineatus too. The rosettes were a bit clearer this time, and the callipers the same as the male above.

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

This is the female caught with the male above.

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