Margarinotus purpurascens

I used Duff initially to identify this but struggled due to an initial lack of clarity as to precisely what the subhumeral stria was. Maybe this should have been obvious (after all it runs below the humerus). I'm still learning what's meant by different anatomical references and humerus is one where I've had to slightly adjust my understanding - in all my first encounters with the humerus in beetle identification it has referred to a raised bump at the basal corners of the elytra, and I took the whole area covered by this bump to be the humerus. This bump (or the place where it would be if it had one) covers an area not a precise point, so I've struggled where keys refer to the humerus as a precise point. Now I realise Duff's introduction diagram does in fact point to a specific point on the corner of the elytra, not the general area which I had previously understood to be the humerus. If I'd realised that first maybe it would have been obvious which stria was the subhumeral one. Anyway, Hackston's key illustrates this nicely (especially if you follow the genus Hister lead having misunderstood it first) and this set me straight.

The red on the elytra was hard to see in life, not at all apparent in some photos from above (unless you squint really hard and apply a healthy dose of imagination). It was more obvious in photos taken from the side and very clear looking at the beetle under the microscope.

Margarinotus purpurascens showing antenna, gunar lobe, fore margin of metasternum and subhumeral stria, North Elmham (Norfolk, UK), 14th April 2022

The next two were found in pitfall traps. One was relatively straightforward to identify:

Margarinotus purpurascens, Wendling Beck Environment Project (Norfolk, UK), 11th-12th May 2025

I would have been surprised if the second had turned out to be anything else but not being familiar with other members of the family I keyed both properly (using Duff). There was a slight problem at the key to subgenus for this one as there was a choice between the sutural stria being absent in the basal half or almost complete. The first dorsal stria was absent in the basal half but the sutural stria extended to around three quarters - so not absent in the basal half or almost complete. Taking it to be absent led to purpurascens but taking it to be almost complete led to marginatus which didn't really fit (e.g. there's no mention of the possibility of red on the elytra, and none of the photos I could find only have this). To make quite sure I checked the aedeagus (though this was only a positive match to purpurascens not a negative match to marginatus as I can't find any illustrations of marginatus aedeagus). It matched the diagram for purpurascens in Duff but I looked at the Beetles of Europe website to see if they had pics of marginatus aedeagus. They don't, but their picture of marginatus (and other species) aedeagus are side views with the median lobe partly pulled out. This view also matches mine so I think it's safe to assume the ID is correct.

male Margarinotus purpurascens showing elytron and aedeagus (flat, close-up of apex and side with median lobe partly pulled out), Wendling Beck Environment Project (Norfolk, UK), 11th-12th May 2025