Melanotus castanipes
I'm not sure about all of the characters but I think the shape of the pronotum (dorsally at least) suggest castanipes, and on this one the genitalia seem to be quite clear. The ratio of the length of the apical 'blade' of the paramere to the whole paramere length comes out at less than 0.23 so clearly pointing to castanipes, assuming I'm measuring the correct distances (I've shown these in red lines below).
, North Elmham, 12-Jun-23 (A) (1).jpg)
, North Elmham, 12-Jun-23 (A) (4).jpg)
, North Elmham, 12-Jun-23 (A) (3).jpg)
, North Elmham, 12-Jun-23 (A) (2).jpg)
, North Elmham, 12-Jun-23 (A) (5).jpg)
, North Elmham, 12-Jun-23 (A) (6).jpg)
, North Elmham, 12-Jun-23 (A) (7).jpg)
male Melanotus castanipes showing pronotum (from above and behind), face and genitalia (3 shots, the last showing red lines where I took my measurements), North Elmham (Norfolk, UK), 12th June 2023
This one seemed to key to castanipes using both Hackston (which basically only looks at the relative lengths of the pronotum and elytra) and Duff (which uses a number of subtle characters). Differences in genitalia aren't exactly enormous and this one was not as the one above, however by my measurements and calculation the paramere blade occupies about 0.27x the length of the paramere, which just about puts it in castanipes range.
, Cathedral Meadows, 2-Jun-21 (A) (1).jpg)
, Cathedral Meadows, 2-Jun-21 (A) (2).jpg)
male Melanotus castanipes showing genitalia, North Elmham Cathedral Meadows (Norfolk, UK), 2nd June 2021
I examined this one at the same time as another Melanotus caught a few days later, and photographed the genitalia side-by-side. At first I thought they were both castanipes but when I reviewed my records and photos recently (having just spent some time looking at other examples) I immediately questioned whether the second was really castanipes. Measuring the relative height of the paramere blade I now consider the second villosus.
, Cathedral Meadows, 2-Jun-21 (L) & Melanotus villosus (m), Bintree Wood, 6-Jun-21 (R).jpg)
genitalia of male Melanotus castanipes, North Elmham Cathedral Meadows (Norfolk, UK), 2nd June 2021 (left) and male Melanotus villosus, Bintree Wood (Norfolk, UK), 6th June 2021 (right)
Neither my notes nor my photos suggest that I dissected this one and I think it may have been before I got the relevant volume of Duff. In that case this ID would have been purely based on the relative lengths of the pronotum and elytra. Not sure how reliable that is in hindsight, but judging from the photos I think the shape of the pronotum fits castanipes.
 (1).jpg)
 (1) - pronotum.jpg)
 (2).jpg)
Melanotus castanipes, Hills & Holes (Norfolk, UK), 21st May 2020