Atomaria nigriventris

This was in a pitfall trap (in propylene glycol, so no live photos). I find the keys to this family's genera can be a bit tricky to follow, but fortunately there aren't many alternatives to check. Once I was happy it was Atomaria it keyed to nigriventris or puncticollis smoothly enough. Duff differentiates these two on the proportions of the elytra and the shininess vs. microsculpture of the metasternum. The proportions are pretty close and with the vagueries of measuring a curved structure (particularly in this case as one of the elytra was loose and had broken off by the time I attempted it) it was too close to call. According to Duff the metasternum should be "shining, without a microsculpture" on nigriventris or "matt due to a microsculpture" on puncticollis. My challenge here was that under high magnification I could see some microsculpture, but at the same time it was clearly shiny. Favouring nigriventris (as the microsculpture wasn't obvious but the shininess was) I checked the genitalia. The spermatheca matches Duff's diagram for nigriventris, differing from puncticollis in having the collum abruptly broadened just before the spermathecal duct.

Hackston (which doesn't require the metasternum to be lacking microsculpture, just shining) provides additional differences which supported my ID as nigriventris. The shape of the pronotum in nigriventris should apparently be "more or less curved inwards just before the hind angles" on nigriventris but not on puncticollis, and so it was on mine (evident under the microscope and in the photo of the whole insect, though strangely not evident on the close-up photo of the pronotum below). Also puncticollis pronotum apparently shows a narrow transverse basal impression that was absent on mine. There is also a difference in size between nigriventris and puncticollis, the latter being larger. Mine measured larger, within range for puncticollis and not nigriventris, however as seems to often be the case with specimens trapped in propylene glycol the head was extended out from the pronotum (with a clear gap between them, joined only by the membranous tissue) and the pronotum similarly from the abdomen. In view of this I'm not worried about the measured size being a little bit too big.

female Atomaria nigriventris showing head, pronotum, antenna, metasternum and spermatheca, Wendling Beck Environment Project (Norfolk, UK), 29th to 31st March 2024