Onthophagus similis

I would have used Mike Hackston's keys to identify the first two here.

male Onthophagus similis, North Elmham (Norfolk, UK), 4th May 2018

male Onthophagus similis, North Elmham Cathedral Meadows (Norfolk, UK), 8th April 2019

This one was swept from heather. Having had two before I was fairly confident it would be an Onthophagus, and probably similis, but by the time I got round to checking it I hadn't seen an Onthophagus for quite a while. I don't think there's anything similar in any other genus, but to be on the safe side I decided to key it out to genus. Using Duff I encountered multiple problems despite it being a short key! Firstly determining whether or not the elytra completely covered the abdomen. In my photo taken from more-or-less directly above, the abdomen was not visible, but examining the specimen the elytra did seem to fall marginally short of the end of the abdomen. I could go either way here, and indeed comparing Duff with Hackston, the two keys demand a different answer to basically the same question. Using Duff you have to go with "Pygidium ± clearly exposed", whereas using Hackston you have to go with "Last segment of abdomen completely covered by the elytra, only visible if viewed from behind." Both provide supporting information that helps make the right decision, though in Duff's case this is only comparison with photos of representatives of each genus. Next problem using Duff was deciding if the claws were simple or appendiculate - there was a basal sinuation which I thought perhaps could be interpreted as a slight appendiuculation, so I wasn't quite sure which way to go, though if you go the wrong way here (to subfamily Melolonthinae) you realise when you read the introduction to the subfamily as Melolonthinae has as evident scutelum. Finally I couldn't see any spurs on the hind tibia (eventually I realised this was because the spurs had broken off both hind tibia - one was visible on one in the photo of the live insect but no longer on the specimen when I examined it). In this case the Hackston key was much simpler and very quickly arrived at the correct genus.

female Onthophagus similis showing head from behind and hind tarsal claw, Buxton Heath (Norfolk, UK), 23rd April 2021