Hydroporus angustatus

I wish I'd annotated my copy of Duff the last time I tried to identify this genus (a palustris) as I had exactly the same issue with this one. I really can't see any way round it - the elytra are translucent and not completely opaque, contrary to Duff's first couplet in the Hydroporus key. Keying this one as translucent led to ferrugineus/obsoletus (both very rare in Norfolk), with ferrugineus fitting best but imperfectly. I'm sure it's not that though. Using Hackston's key it keyed straighforwardly to angustatus (one of the commonest species in the genus) and it also leads to angustatus using Duff if we ignore the fact that the elytra aren't completely opaque.

This beetle was 3.5 mm long.

female Hydroporus angustatus showing base of elytral epipleuron, close-uo of base of pronotum and elytra, prosternal process, hind femur, hind coxal process, elytra showing translucency (2 shots), elytral puncturing, pronotal/elytral sides angle and front tarsi, Hoe Rough (Norfolk, UK), 23rd July 2023

This similar-looking beetle came to light at the same place a month after the one above. This time I remembered the issue with the translucency of the elytra so it was a bit quicker to deal with. That said, it wasn't easy. Keying it to genus there were a couple of quibbles - the prosternal process was clearly sharply pointed rather than rounded but I couldn't tell that the margin (clear at the sides) stopped short of the apex. It was hard to see the posterior border of the hind coxal processes clearly enough to tell if it was straight at the centre or if it had a median notch - but if it did have a median notch it was not as clear as shown in the diagram for that option. Once I got to Hdyroporus it was more straightforward once I'd checked it didn't key properly if I took the translucent elytra option. This one was a shade under 3.2mm.

female Hydroporus angustatus showing head and pronotum, prosternal process, hind femur, hind coxal process and elytral microsculpture, Hoe Rough (Norfolk, UK), 23rd August 2023

I'm slightly worried about this ID but I think it must be right. It was very slightly below the bottom end of the size range for angustatus as given in Duff, but at between 2.6mm and 2.7mm it was very close. Perhaps more concerningly the punctures on the elytral disc were more widely spaced than they should be. But using Hackston it keys to angustatus (other than being small) and as it was fully-winged it can't be scalesianus (which is also even smaller). There seem to be multiple reasons why anything else doesn't match, so I think it's got to be angustatus.

female Hydroporus angustatus showing elytral epipleuron, pronotum & base of elytra, hind femur, coxal processes, prosternal process, elytral microsculpture, fore tarsus and elytral puncturing (elytra closed & lifted), Wendling Beck Environment Project (Norfolk, UK), 6th September 2023

This one was just over 2.8mm long.

Hydroporus angustatus showing elytral microsculpture, pronotal/elytral angle, prosternal process and coxal processes, Wendling Beck Environment Project (Norfolk, UK), 8th September 2023

This one was 3.0mm long. It had slight transverse impressions on each side of the pronotum which I hadn't seen on previous examples of the species, at least not as clearly. For this to have made a difference to the ID these impressions would have needed to be longitudinal not transverse.

Hydroporus angustatus showing pronotal impression and prosternal process, Wendling Beck Environment Project (Norfolk, UK), 11th September 2023