Atrecus affinis (Paykull, 1789)
This widespread Palaearctic species is the only member of the genus to occur in the U.K., it is generally common in suitable habitats throughout Europe, extending into Turkey, Iran and eastern Siberia, and here it occurs throughout; to the far north of Scotland. The typical habitat is mature woodland and wooded parkland etc. where there is plenty of wood in an advanced state of decay; both larvae and adults occur under loose bark on the trunks and fallen timber of a wide range of deciduous and coniferous trees, they often occur in damp conditions under old bark covered in moss and litter, or where grass etc. is growing over old logs. We have recorded them from birch, beech-several times from logs on calcareous grassland, oak, pine and fir. Adults are occasionally present in extractions of saproxylic fungus but they do not appear to be closely associated with fungi in the wild. In South Hertfordshire we have recorded them throughout the year; during the spring often in small aggregations and sometimes in cop. but otherwise as pairs or single specimens, and we have yet to find them active nocturnally. This very distinctive species is readily recognized in the field by the characteristically coloured body and pale appendages, the only confusion might be with various Othius but here the abdomen is uniformly coloured and the mandibles are not so produced.
6-7.5mm. Head slightly transverse, shiny black with strong linear microsculpture that is obvious at X40, characteristic long, straight-sided mandibles and small, weakly convex and circular eyes. The clypeus has a single longitudinal furrow behind each antennal insertion and the labrum is narrow and deeply emarginate. The surface has scattered micropunctures and a distinctive pattern of larger setiferous punctures; 4 behind each eye and 4 in a transverse row across the vertex from the centre of the eyes. The temples are long, rounded and narrowing to a wide and swollen neck. Antennae inserted dorsally between the outer margins of the mandibles, the basal segment as long as the next 2, and the rest becoming gradually wider towards the apex. The palps are long and slender with the terminal segment pointed. Pronotum elongate and slightly narrower than the head, red although often darker laterally, the surface with linear microsculpture and 4 large punctures; 2 near the anterior margin and 2 near the middle, and finely bordered throughout. Elytra shiny black with the base red, weakly rugose and finely punctured but without microsculpture, the sutural slightly overlapping and strongly bordered. Abdomen strongly bordered, red with segments 4 and 5 darker, sparsely punctured and pubescent, and each segment with a long seta before the hind angles. The legs are robust and long, all tibiae with a strong spine on the inner apical angle, meso-tibiae with strong spines along the outer edge and the meta-tibiae with similar spines towards the apex. Pro-tarsal segments dilated, more so in the male.