Cercyon ustulatus (Preyssler, 1790)
POLYPHAGA Emery, 1886
HYDROPHILOIDEA Latreille, 1802
HYDROPHILIDAE Latreille, 1802
SPHAERIDIINAE Latreille, 1802
Cercyon Leach, 1817
Dicyrtocercyon Ganglbauer, 1904
The only member of the subgenus Dicyrtocercyon Ganglbauer, 1904, this species is locally common throughout Europe but for various areas in the extreme north and south, it occurs across Asia Minor and Russia and extends into Siberia and is now established in the United States and Canada following accidental introductions from Europe. It is generally common across England and Wales including Anglesey and the Isle of Wight although absent from most of the West Country, and more local across Ireland and southern Scotland. Adults have been recorded throughout the year and are active over a long season from early spring, peaking in abundance in late spring and late summer, they occur in a range of permanently damp and wet habitats, most often among decaying vegetation in marshes and wetland margins but also in dung and compost and they have been recorded from various mammal nests including those of beavers and common voles, in which adults were found overwintering. Breeding occurs in the spring and early summer and both adults and larvae are thought to be saprobic, the larvae developing among decaying organic matter, adults often occur in numbers, they disperse by flight in the evening and have been recorded at MV, actinic and flight-interception traps. Adults are easily sampled throughout the year by inspecting leaf-litter in marginal habitats, they often occur in extraction samples taken from various damp habitats and may occur in large numbers among flood-refuse.
2.6-3.4mm. Broadly-oval and very convex, in side view the pronotum and elytra form separate curves, where they meet forming a distinct angle, and this is diagnostic for the species. Body shiny black, lacking microsculpture but finely and quite densely punctured throughout, elytral apices red or brown separated by dark sutural margins, legs dark brown to black, usually with the tarsi paler, palps pale brown and antennae ;ape brown with a darker club. Head smoothly convex with eyes that more or less follow the outline, anterior clypeal margin straight, punctation similar to that on pronotum but a little denser than those on the elytra. Pronotum, from above, broadest at perpendicular posterior angles and narrowed to slightly protruding anterior angles, lateral margins curved and basal margin sinuate, surface even but for a tiny groove at the centre in front of the basal margin. The mesosternal process, often helpful in Cercyon identification, is long, narrow and pointed and touches, or almost touches the obtusely-angled anterior margin of the metasternum. Each elytron with 10 well-impressed and finely punctured striae extending from before the basal margin to the apex, interstices finely and randomly punctured throughout, mostly flat but weakly convex towards the apex. Front tibiae rounded apically, middle and hind tibiae truncate, all with strong apical spurs. Tarsi 5-segmented; the basal segment of middle and hind tarsi longer than the others, claws smooth and without a basal tooth.