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Cercyon obsoletus (Gyllenhal, 1808)

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POLYPHAGA Emery, 1886

HYDROPHILOIDEA Latreille, 1802

HYDROPHILIDAE Latreille, 1802

SPHAERIDIINAE Latreille, 1802

Cercyon Leach, 1817

Cercyon Leach, 1817

This widespread western Palaearctic species extends east from Europe as far as the Black Sea; it is recorded from Algeria, Tunisia, Morocco and Iran, and is present on the Azores and the Canary Islands. In Europe it is generally distributed from Portugal to Italy and Greece in the south, and it extends north into the UK and the Baltic countries where it reaches the Arctic Circle in Sweden and Finland. It is sometimes said to have been introduced into the Neotropical region, but this is in error. With the exception of the West Country, the species is widespread and generally common across southern and central England and Wales, rather less so further north into Southern Scotland, and very local and rare in the Scottish Highlands and Northern Ireland. Adults are mainly associated with herbivore dung (although they are sometimes said not to occur in that of sheep) and may occur in large numbers on dung pasture during the summer, but more generally they may occur among decaying vegetation in most situations, they are occasionally recorded at carrion,  and are widely recorded from decaying seaweed on the continent. They have been recorded year-round; overwintering in the soil or under old dung etc, and active from April until September or October, peaking in abundance during April and gradually declining into the autumn. They often fly above dung pasture and are occasionally attracted to light traps though usually only in small numbers. The biology is unknown. Adults may be extracted from dung through the spring and summer, and they sometimes occur in flood refuse or among decaying litter during the winter.

Cercyon obsoletus 1

Cercyon obsoletus 1

Cercyon obsoletus 2

Cercyon obsoletus 2

3.5-4.2 mm. Broadly-oval and very convex, body glabrous, dull black with the elytra becoming pale apically, sometimes extensively so but see below castaneipennis, ventral surface black, often with contrasting dull brown elytral epipleura, antennae and palps dark grey to black, legs brown. Head transverse with small eyes that follow the outline and long converging temples, clypeal margin smoothly curved, surface weakly convex and finely and densely punctured throughout. Maxillary palpi about as long as the antennae. Antennae 9-segmented with an oval and compact club. Pronotum transverse, broadest across acute posterior angles and narrowed to slightly projecting anterior angles and a curved apical margin, lateral margins finely bordered, basal margin sinuate laterally. Pronotal surface evenly convex and finely and densely punctured, without microsculpture.  Scutellum elongate, triangular and punctured as the adjacent pronotum. Elytra broadest before the middle, smoothly curved from obtuse shoulders to a continuously rounded apical margin, each with ten impressed and punctured striae complete to the apex, interstices slightly convex and densely punctured throughout. Elytral margin with a small oblique impression just before the base, this is diagnostic among our UK species. Legs short and robust, tibiae with numerous external spines, and paired apical spurs. Tarsi with five simple segments.

C. castaneipennis Vorst, 2009, a widespread European species which extends north into Fennoscandia, is very similar to the present species and occupies similar habitats but has not yet been recorded from the UK. It is similar in size and has the oblique sub-humeral impression, but specimens usually have more extensively brown elytra, often with only the area around the scutellum darkened, the dorsal punctation is fine but less dense compared with obsoletus, and the elytral striae are better defined, especially towards the base, and the antennal club is slightly longer. The species is very likely to occur here, see Vorst, 2009.

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