Rhantus frontalis

This was keyed using Duff and checked using Hackston. The underparts were more uniformly yellow than is suggested by Duff's "bicoloured" but there is a bit of variation in the darkness (and some of the sutures are darker). The photo below of the underside of the beetle looks a little redder (so less yellow) than it really looked. The mark at the centre of the pronotum was obvious looking at the specimen but is less obvious in the live photos where it is more obscure among the dark colours showing through from below at the front and back of the pronotum. The mark is only slighly transverse which isn't a problem using Duff's key but it's described as transverse in Hackston's key. The beetle was 10.8mm long.

Rhantus frontalis, North Elmham (Norfolk, UK), 23rd August 2023

This was keyed as Rhantus frontalis at a time that I had had little experience with the genus, or indeed with using keys to identify beetles at all. I think the ID was at least partly based on the colour of the underside but unfortunately I didn't take any photos of this. When I came to reviewing all my records of Dytiscidae in early 2023, before I'd caught the specimen shown above, I looked at the pattern of the pronotum and head and compared it with other photos of frontalis online, and became unconvinced that my original ID was correct - it looked suspiciously like suturalis. Having not had a repeat of frontalis I decided that my original ID was unsafe. But having now had another, and realising that the pronotum pattern is not always quite as it looks on live photos, I am more inclined to trust my earlier efforts. I think you can just make out the dark mark on the disc of the prontoum, only slightly separated from the apparently dark area at the rear, and if I'm right then it's more transverse in shape than my later specimen above. Coincidentally (or not) it was on exactly the same date as the 2023 individual.

Rhantus frontalis, North Elmham (Norfolk, UK), 23rd August 2017