Berosus affinis
My first Berosus proved reasonably straightforward to key out.
 (1).jpg)
 (2).jpg)
 (3).jpg)
 (4).jpg)
 (5).jpg)
 (6).jpg)
 (7).jpg)
 (8).jpg)
male Berosus affinis showing pronotal punctures, underside of abdominal sternites, mesosternal ridge and aedeagus, Wendling Beck Project (Norfolk, UK), 29th October 2022
 (1).jpg)
 (3).jpg)
 (2).jpg)
 (5).jpg)
 (7).jpg)
 (6).jpg)
male Berosus affinis showing pronotal punctures, underside of abdominal sternites and mesosternal ridge, Wendling Beck Project (Norfolk, UK), 8th September 2023
With its head held down so it's only just visible from above you could easily miss the lovely irridescent metallic colouring on it.
 (2).jpg)
 (1).jpg)
 (3).jpg)
 (7).jpg)
 (4).jpg)
 (6).jpg)
 (5).jpg)
 (9).jpg)
 (10).jpg)
male Berosus affinis showing head and pronotum, hind tarsus and swimming hairs, apex of apical sternite, apex of elytra, rear slope of elytra and mesosternal keel (head is to the right), Wendling Beck Project (Norfolk, UK), 12th August 2026