Bisnius fimetarius (Gravenhorst, 1802)
This is one of the most widely distributed and common Palaearctic members of the Staphylininae, occurring across North Africa and through Asia Minor to Kazakhstan and India, and from Portugal to Mongolia and north throughout Scandinavia; it was first recorded from China (Ningxia) in 2010 and from Canada in 2011 where it is now established in Quebec and Newfoundland. In the UK it occurs throughout the mainland and on most of the islands although records are sparse in the West Country and from Scotland generally. Both adults and larvae are predatory, feeding upon the early stages of other insects etc. Adults occur throughout the year, peaking in late summer and autumn, in a wide range of habitats; decaying vegetation generally, compost heaps, dung, accumulated leaf-litter and decaying fungi. They are known to breed in dung of all kinds and on warm sunny days may be netted in flight over dung pasture, they are also nocturnal, especially in the autumn when very large numbers may be present among decaying large terrestrial bracket fungi, they may remain active through the winter and into the spring when sieving samples over a sheet may reveal hundreds of specimens. More generally they occur in extraction samples from litter, tussocks or decaying vegetation from damp habitats through the autumn and winter. During warm spring and summer days they occasionally occur when sweeping dense vegetation bordering grassland and wetland and at this time they move rapidly and fly readily.
This rather nondescript species is likely to be confused in the field with many other medium-sized staphs, especially species of Philonthus from which they can be distinguished by the elongate second pro-tarsomere, and Quedius from which they differ in having 4 punctures either side of the pronotal disc. Identification is straightforward as the wide depression between the antennae is unique among UK species.
5.5-7mm. Entire body dark or with the margins of the scutellum and the apical margins of the abdominal segments pale, and the elytra usually have a metallic green lustre. Antennae dark although often obscurely lighter at the base, mouthparts and legs pale. Head quadrate with small weakly convex eyes and long temples which are near parallel in the male and taper towards the base in the female, vertex with weak cellular microsculpture and large lateral punctures; two anterior series more widely separated than the distance from each corresponding eye. Antennomeres 4-11 quadrate to slightly elongate and finely pubescent. Pronotum elongate and parallel-sided, with a series of 4 large punctures either side of the middle and weakly microsculptured. Elytra distinctly broader than pronotum; elongate and broadened from base to apex, strongly and moderately densely punctured and with recumbent overlapping pale pubescence. Abdomen shiny, lacking microsculpture and faintly metallic, finely and sparsely punctured and with recumbent overlapping pubescence. Legs pale brown or variously darker; in most specimens the tibiae and tarsi are darker than the femora. Tarsi 5-5-5. Basal metatarsomere shorter than segments 2-4 or segment 5. Claws well developed but shorter than fifth segment.