Acrotrichis grandicollis (Mannerheim, 1844)
POLYPHAGA Emery, 1886
STAPHYLINOIDEA Latreille, 1802
ACROTRICHINAE Reitter, 1909
ACROTRICHINI Reitter, 1909
Acrotrichis Motschulsky, 1848
Ctenoptreyx Flach, 1889
This species is locally common throughout much of the central and Northern Palaearctic region; it extends from Europe to the far east of Russia, China and south into the Oriental region. In Europe it occurs from lowlands to the alpine zone throughout the region, it is present on most of the Mediterranean islands and across north-western Africa, to the north it extends to the UK and occurs throughout the Baltic countries, reaching far above the Arctic Circle in Scandinavia, although it is not known from the Atlantic islands, Faroe or Iceland. In the UK it is widespread though sporadic throughout England and Wales, including Anglesey and Man, and very local and rare in Scotland and Northern Ireland, although in Scotland it occurs in the highlands and the Inner Hebrides and so it may well be more widespread than is realized. Adults are present year-round; they overwinter among decaying organic matter or under bark and are generally active from April until October, peaking in abundance during May and June and again in September, although they will often be found active during mild winter spells. Adults occur in a wide range of situations, they may be common among compost and older and drier herbivore dung e.g. of Horse, cattle, sheep and deer, but they also occur among decaying deciduous wood and under dead bark. In Northern Europe they are often recorded from oak (Quercus L.) and ash (Fraxinus L.), from various decaying fungi including Laetiporus sulphureus (Bull.) Murrill (1920) and Cerioporus squamosus (Huds.) Quélet (1860), and under old carrion. Adults will appear through the warmer months when sieving compost etc., and during the winter they often appear in extraction samples, e.g. we have recorded them on two occasions from old bird nests. They tend to occur in numbers at any time but during the summer they often occur in abundance and they may swarm when sieving compost or decaying leaf-litter etc., but they often occur among numbers of other ptilids and so specimens will need to be examined carefully.
0.9-1.0 mm. Broadly-oval and distinctly constricted at the pronotal/elytral border, dorsal surface black with a distinct metallic reflection or sometimes with the elytral apices paler. Body rather densely pubescent, finely and randomly punctured throughout and with distinct, though often faint, microsculpture. Antennae dark grey to black with two basal segments dark brown, legs pale brown throughout. Head transverse and evenly curved in front of convex eyes that follow the outline. Antennae less than half the body length, 11-segmented; the two basal segments broad and almost parallel-sided, 9-11 elongate and slightly broadened, giving the impression of a long and loose club, intermediate segments not constricted towards the base. Pronotum transverse and broadest slightly behind the middle, narrowed to obtuse anterior angles and slightly (but always discernibly) sinuate before produced and acute posterior angles, lateral margin with a single erect seta about the middle (sometimes one or both may be broken but this is an important character), basal margin strongly curved towards the posterior angles. Scutellum triangular and proportionally very large. Elytra quadrate or nearly so, evenly but not strongly narrowed from rounded shoulders to truncate apical margins, lateral margins each with two outstanding setae (important character). At least two tergites are visible beyond the elytra; the pygidium with three sharp teeth on the apical margin. Legs short and slender with femora not visible in normal setting and tibiae with fine setae along the margins. Tarsi 3-segmented.
Two subgenera of Acrotrichis are represented in the UK: Acrotrichis s.str with 24 species which are often notoriously difficult to identify, and Ctenopteryx Flach, 1889 which includes the present species and one other, A. sanctaehelenae Johnson, 1972. Ctenopteryx may be distinguished by the presence of several outstanding setae along the lateral margins; one on the pronotum and two on the elytra. In A. sanctaehelenae the dorsal surface is not metallic and the pronotal margin is straight before the posterior angles.