Cercyon unipunctatus (Linnaeus, 1758)

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POLYPHAGA Emery, 1886

HYDROPHILOIDEA Latreille, 1802

HYDROPHILIDAE Latreille, 1802

SPHAERIDIINAE Latreille, 1802

Cercyon Leach, 1817

Cercyon Leach, 1817

This native Palaearctic species is has now Holarctic since becoming established and widespread in the United States and Canada following introductions during the middle of the 19th century, it is generally common throughout Europe, occurring to the far north and also in Iceland but is absent from the extreme south, to the east it extends across central and northern Asia to the north of China and is also present in the north of Japan. Here it is locally common across Wales and central and eastern England north to Nottingham, and more sporadic and scarce in the West Country and further north to Orkney. Adults occur year-round and are active over a long season, from March until October, peaking in mid-summer, and they may also appear during mild winter spells, they fly well, are crepuscular and nocturnal and they are attracted to light, sometimes in large numbers, and may occasionally swarm over dung pasture during the hottest weather. They may be found in open habitats among herbivore dung throughout the summer, particularly that of horse and cattle, and this is probably the most reliable host material from which to sample them, but they also occur more generally in decaying organic matter in a wide range of situations; they occur among decaying seaweed and salt marsh litter, decaying vegetation on wetland margins and among decaying fungi and dung just about anywhere, they are also synanthropic and may be abundant in garden compost, manure heaps and old hay and straw bales. Mammal and bird nests in a range of situations may also host the species; we have recorded them from old woodpecker and moorhen nests and more generally a range of nests, including those of some subterranean mammals, have been recorded hosting the species.

2.4-3.4mm. Distinguished among our UK fauna by the yellow elytra which usually have a large and well-defined dark mark about the centre, this is occasionally reduced or absent but the sutural interstice is always darkened in the apical half, this will separate the present species from C. quisquilius (Linnaeus, 1761), with which it is often found in dung. Dorsal surface shiny and finely punctured, with no contrast between the forebody and elytra, head black, pronotum black with narrow paler margins, scutellum black, elytra (at least in UK specimens), elytra as above and ventral surface black or dark brown, usually with the posterior margin of the ventrites paler. Antennae pale with the club dark brown, maxillary palps brown with the terminal segment darker, legs dark reddish-brown. Head with small weakly convex eyes and smoothly convex vertex and frons; the frontoclypeal suture not visible. Pronotum transverse, broadest across the base and narrowed from perpendicular posterior angles to slightly protruding anterior angles, lateral margins with a fine border that may continue for a short distance onto the basal and apical margins, surface smoothly convex; without fovea or impressions. Metasternum without femoral lines. Outer elytral interstice smooth beneath the humerus; without a short transverse ridge, all interstices flat or weakly convex, finely and randomly punctured and lacking microsculpture, striae much narrower than the interstices and impressed and punctured to the apex. Front tibiae smoothly rounded apically-a feature that will separate the present species from C. littoralis (Gyllenhal, 1808), a coastal species that varies widely in colour. The elytra of UK specimens of unipunctatus are usually of the typical maculate form but on the continent they vary widely, from almost entirely yellow to substantially dark where the central marking extends to the shoulder and broadly to the apex.

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