Atomaria nigrirostris

This should have been a fairly straightforward ID but I managed to get confused! For some reason that I cannot now fathom, instead of searching for a transverse impression at the base of the pronotum I was searching for a transverse keel (according to my notes). Both Duff and Hackston describe it as an impression not a keel so I can't imagine why I got mixed up. Anyway, needless to say I couldn't find a transverse keel, and taking it as not having a transverse impression I couldn't get it to key at all. In desperation I looked up what the commonest Atomaria was (linearis) and tried to work backwards from there. There were problems with my interpretation of the elytral hairs but I managed to shoe-horn it through and concluded, albeit very tentatively, that it must be linearis.

Given the difficulties in getting there I thought it wise to check the genitalia matched linearis. Duff shows diagrams of the spermatheca for all the various Atomaria species and of all the diagrams the only one that was a really good match was the one for nigrirostris. But the next closest was linearis. The diagram for nigrirostris was the only one showing the coiled collum at the bottom as mine had. There was also a difference in the diagrams between linearis and nigrirostris in the shape of the middle section (the nodulus?). On linearis this section was shorter, and starts thick as it curves round from the top (cornu I presume), fairly abruptly narrowing towards the collum which starts before its level with the tip of the cornu. On nigrirostis, and on mine, the central section was longer, passing the tip of the cornu and starting to curve round before it turns into the collum (I'm taking the collum to start where it's evenly narrow). It tapered much more gradually. There's usually some subtle variation between individuals when it comes to spermatheca shape and I don't always expect an exact match in every respect, but here it was pretty much an exact match to nigrirostris and a couple of significant differences from linearis.

So in conclusion I decided it was either a nigrirostris that was lacking a transverse keel (!) on the pronotum or it was a linearis that had a spermatheca that is a much better match for nigrirostris. I noted that the diagram of nigrirostris habitus (Fig. 58.9) in Duff showed a much more rounded front of the pronotum compared to my beetle, but photos of nigrirostris online showed the same shape as mine (as did linearis).

The habitat described for nigrirostris in Hackston is completely different from that given in Duff - I don't know which is right but neither match where I found this. The habitat for linearis seems a bit more like where I found it - in a pitfall trap beside a pylon in the middle of an arable field (though no haystacks here and I don't think the current crop is beet).

This was all very unsatisfactory but I was unable to resolve it fully and left it as unidentified (publishing it here under linearis but not claiming it as such). Months later I found another Atomaria nigirostris and this time didn't switch impression for keel in my brain! The impression was still difficult to see from some angles (it just looked like the inner edge of the basal bead), but fairly obvious from other angles. I can only conclude that I didn't see the basal trasverse impression because I was looking for a basal transverse keel! It's not altogether clear in the photos I took but I think I can see it there. At least I now I understand why the genitalia looked like nigrirostris despite it not having a basal tranverse keel (!) on the pronotum!

female Atomaria nigrirostris showing spermatheca, elytral hairs, pronotum and antenna, Wendling Beck Environment Project (Norfolk, UK), 17th-18th May 2023

The next one was much simpler as I looked for the right thing this time (although I still managed to type 'keel' when I meant 'impression' in my first draft of the next sentence!). The basal impression wasn't immediately obvious but a slight change of angle and it popped into view. It was closer to the basal bead than I'd imagined and at the wrong angle it was hard to differentiate from the edge of the bead. In fact my photos have failed to pick up the impression at all - what looks like a large rounded depression near the base of the pronotum is an artefact of the photo - it was not real. I had intended to confirm this genitalically but was unable to find the spermatheca so could not do so, but I think the external characters (as seen if not as visible in the photos) are diagnostic enough.

female Atomaria nigrirostris showing head from side, pronotum and antenna, North Elmham (Norfolk, UK), 15th August 2024