Recently assimilated into the Staphylinidae, the pselaphines will probably carry on being called pselaphids by many coleopterists for a while yet. They share the short elytra typical of normal staphs but have a distinctive, rather ant-like appearance, tough, heavily-sclerotized exoskeletons, and short, inflexible abdomens. They are small beetles ranging from 0.9 to 3.5 mm body length. There are 54 British and Irish species.
In size and ‘jizz’ they are confusable with the Scydmaeninae (32 species), another recently assimilated subfamily of the Staphylinidae. However, scydmaenines have the abdomen covered by their elytra except sometimes for the tip of the last segment.
The main identification reference is Pearce’s (1957) RES Handbook, out of print but available to download as a searchable PDF here. I am currently working (very slowly!) on new keys to this subfamily and will post test keys here in due course.
Pearce, E.J. (1957). Coleoptera (Pselaphidae). Handbooks for the identification of British insects, vol. IV, part 9. London: Royal Entomological Society.