Agathidium nigrinum Sturm, 1807
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POLYPHAGA Emery, 1886
STAPHYLINOIDEA Latreille, 1802
LEIODINAE Fleming, 1821
AGATHIDIINI Westwood, 1838
Locally common in France and parts of Northern Europe but otherwise generally very local and scarce, this species occurs widely from the Pyrenees to Italy, Ukraine and the Caucasus in the south, and north to the UK, Denmark and the Baltic countries where it extends north to the Arctic Circle. In the UK it is locally common across Southern and Central England and Wales and much more local and scarce further north to the Scottish Highlands. The typical habitat is damp and shaded woodland with plenty of decaying timber but the species is also likely to occur on isolated trees in hedgerows or even domestic gardens. Adults are associated with a wide range of fungi and slime moulds on both broadleaf and coniferous trees; frequently on beech (Fagus L.), Aspen (Populus tremula L.), birches (Betula L.), oaks (Quercus L.), willows (Salix L.), hazel (Corylus L.) and elms (Ulmus L.), also from firs (Abies Mill.) and pines (Pinus L.) among others. They are active on the surface of trunks and fallen timber at night and may be found among damaged bark or soft wood beneath or near to fungal sporocarps but they often wander and may be found (often in pairs) on litter bins or posts in wooded parkland. Larvae are likely to develop on slime moulds, feeding on spores, plasmodia both, as is usual in the genus but specific associations are not known. Adults have been recorded throughout the year but only rarely so and usually among extraction samples during the winter; they are active from March until late in the autumn and peak in abundance during May and June and again in October or November. Sampling adults usually involves searching trees etc. at night or searching under bark at any time but they also occur among accumulated damp leaf-litter and, during spring and early summer, might be expected from flight-interception traps.
3.2-4.7 mm. Body globose, almost spherical when rolled-up, extended strongly discontinuous in outline and the body sections separately-rounded in lateral view, body shiny and without microsculpture; black to reddish-brown or dark with paler borders to the pronotum, antennae dark, often with some basal and apical segments paler, legs black to brown. Head transverse, broadest behind weakly convex eyes and truncate anteriorly, surface with double punctation; fine punctures with much finer and denser punctures in between, vertex with an arcuate impression behind the eyes and clypeus not delimited by a suture. The post-ocular ridge is at least one-third as long as the eye and very shallowly acute where it meets the temple (from above.) Antennae 11-segmented; segment seven distinctly wider and longer than segment eight, and segments nine to eleven form a long and loose club. Pronotum widely transverse, broadest across rounded posterior angles and narrowed to slightly projecting anterior angles, surface smoothly convex and double-punctured but more finely so than the head. Mesosternum short and curved, sometimes weakly raised but without a median ridge. Metasternum smooth, without femoral lines. Elytra strongly convex and almost circular when viewed directly from above, lateral margins finely bordered, humeral angle rounded and distinct; about 50 degrees with respect to the lateral margin, surface with a partial sutural stria towards the apex but otherwise without striae, double punctured, a little more strongly so than the pronotum. Legs short with femora hardly visible in normal setting, tibiae gradually broadened from the base to truncate apices, with ridges of fine hairs dorsally and ventrally and a single very fine apical spur. Tarsi 5-5-4 in males, 5-4-4 in females, all segments simple, basal protarsomeres weakly dilated in males.