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. (Looking back at this after further experience with the genus I am not sure why I did not pay closer attention to the tarsal claws - the photo appears to show broadened tarsomeres which I now realise would make it a male, so the relative length of the claws would have been helpful but unfortunately I can't make these out clearly enough in the photos. I've added re-edits of two photos cropped to show the fore tarsi - the claws look unequal but the angle is such that I'm not completely sure).

male 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. (Looking back at this again after gaining some more experience with the genus I think I can see enough of the front tarsi in the photos to sex it as a male. The claws aren't completely clear but I think they look ok for frontalis. I've now added a re-edits of both photos cropeed to make the foretarsi easier to see).

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

In 2024 I had a problem with the way I preserved (or rather failed to preserve) my Rhantus specimens between catching them and identifying them. The first two I caught in my garden moth trap were so badly mouldy, wet and disintegrating that it was impossible for me to identify them. This one had a little dry mould too but wasn't nearly so bad. A couple of legs had come off but most of the body was clearly visible and appeaared to have retained its natural colours so far as I could tell. It was 11.4mm long and has a dark mark in the middle of the pronotum so should be either suturalis or frontalis. The yellowish sternites rule out suturalis. According to Duff the front claws are very different in length in male frontalis whereas this one's front claws were only slightly different in length. I assume therefore that it was a female, though I still hadn't worked out how to sex these (update: I now see Duff's introduction to the genus tells us that male foretarsomeres 1-3 are broadened in males which I think confirms this is a female).

The black at the centre of the pronotum isn't visible in the live photo (which suffers from flash reflection here) and I wondered whether I might be seeing the shadow of the less opaque structure beneath the pronotum rather than an actual dark mark on the pronotum. If that were the case it would key to exsoletus, but I don't think it is (exsoletus is meant to be quite a bit smaller).

female Rhantus frontalis showing pronotum, underside and front claws, North Elmham (Norfolk, UK), 12th August 2024

At last in September 2024 I caught a male frontalis which was altogether more satisfying. The sternites were much more extensively black than I had imagined (and compared to previous examples) - but keyed to frontalis straightforwardly and supported by the fore tarsal claws.

male Rhantus frontalis showing sternites and front claws (2 views), North Elmham (Norfolk, UK), 2nd September 2024