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 beetle which I keyed to Atomaria nigirostris and this time didn't switch impression for keel in my brain! I concluded that the first time the only reason 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

I had previously included the second beetle referred to above on this page, but when I looked at this page again while I was examining another Atomaria it was immediately obvious that I had completely stuffed up the second beetle. On that one I'd not found the genitalia but had somehow managed to key it to Atomaria nigrirostris, however looking back at the photos this was clearly incorrect and I'm surprised I didn't realise that at the time. According to my notes it keyed quite neatly to Atomaria nigrirostris using the Cryptophagidae key, but looking at the photos now I'm all but certain that it wasn't even a member of this family - I'm pretty sure it was in fact a member of the Latridiidae family. I have therefore removed the photos from this page and am now treating that record as unidentified.