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