Dytiscus dimidiatus

Most references don't show a vernacular name for this species though I see Brock and iNaturalist have it as King Diving Beetle, which is perhaps a worthy title for such a monster.

Perhaps the absence of a pale border to the rear of the pronotum should have been the first clue that this was not Dytiscus marginalis, the only Dytiscus I've recorded in my garden before. But it was the shape of the hind coxal processes that first alerted me to the possibility, being clearly more rounded. Using Adrian Chalkey's guide to great diving beetle identification this is quickly and easily identified as dimidiatus based on the shape of the coxal processes and the colour of the underparts, supported by the size (a whopping 37mm) and the pronotum pattern. Interestingly Duff doesn't distinguish between marginalis and dimidiatus on shape of the coxal processes, showing dimidiatus as being more pointed as in marginalis. Chalkley does says they can be more elongated, though not really pointed, in dimidiatus, so maybe Duff is playing it safe to allow those with more marginalis-like processes not to be overlooked? I also struggle with Duff's description of the width of the pale bands on the fore and hind margins of the pronotum. Duff says these should be half the width of the median black band on marginalis and no more than a third on dimidiatus etc. Firstly, if I didn't know better, I would assume he was saying that each of the fore and hind margins were half the width of the black on marginalis, rather than the total width of the fore plus hind margins came to half the width of the black. He doesn't make it clear that he's summing the two, but I assume he must be as that's the only way I can get the pale fore and hind margin widths to come to anywhere near half the width of the black on any marginalis I've ever seen myself either in life or in other people's photos. Even then I'm not convinced it's as much as half. Clearly the relative breadth of the pale fore and hind margins of the pronotum is an important ID character, but I don't think the difference is described so well here.

There aren't a huge number of records of Dytiscus dimidiatus in Norfolk and judging from the records plotted at the NBN Atlas this may be a particularly significant record in this part of Norfolk - the map shows no records anywhere in or close to the Wensum catchment area.

male Dytiscus dimidiatus showing head & pronotum, hind coxal processes, underside, hind tibia & tarsus, underside of foreparts and adhesive pad, North Elmham (Norfolk, UK), 28th September 2023