Hydroporus palustris

I found this, my first example of what is apparently a very common species, quite difficult to identify using Andrew Duff's key. There were a few challenges but the main one was the translucency of the elytra. The whole elytra were translucent - if I detached an elytra and held it 2-3 mm above something I could very easily see through it to what was underneath, even when I looked through the black-pigmented parts. According to Duff, the elytra of most species of Hydroporus, including palustris, are opaque. Only two are meant to be translucent throughout, namely obsoletus and ferrugineus, both very rare in Norfolk. As well as being rare, photos of these species online didn't really look like my insect, and what's more my specimen didn't satisfactorily key to either. After re-attempting the key by selecting less convincing alternatives at various couplets in both genus and family keys I got nowhere, until I tried keying it as if it had opaque elytra. That way led to palustris, one of the commonest species in the genus and one where photos online were a good match for what I had in front of me. Surely this was the solution despite the translucent elytra? I couldn't check the aedeagus to remove doubt as it was a female, but I was eventually happy enough to name it as palustris after using Mike Hackston's key and that led more-or-less straight to palustris. I say more-or-less because I found it very hard to see some of the details required on the process of the prosternum - also a problem when using Duff's key. To see that detail using the dissecting microscope was pushing it to its limits and my attempts at the end of the process to clear it and view under the compound microscope didn't really add much.

I have subsequently encountered the same problem with the translucent elytra of commoner Hydroporus (this time with angustatus) - I think it must be a mistake in Duff's key.

female Hydroporus palustris showing process of prosternum, Billingford (Norfolk, UK), 25th April 2022