Deinopsis erosa (Stephens, 1832)

This tiny rove beetle occurs sporadically and is generally rare across central and northern Europe from lowland to low mountain altitudes, extending south to northern Italy, Romania and Bosnia and north to the UK and above the Arctic Circle in Fennoscandia, to the east it occurs in western Russia and probably extends into Siberia. Only one other member of the genus occurs in Europe, D. pulawskii Klimaszewski, 1979, is present in the southeast and sympatric with the present species in Bosnia, the genus is otherwise Holarctic with the greatest diversity in North America, two species occur in Japan, two in Asia and a single species is known from Australia. D. erosa is local and generally scarce across southern England from the Severn Valley to the Humber, it occurs in a range of permanently damp habitats such as among decaying vegetation in wetland margins, marshes and peat bogs but also among litter in wet woodland and in damp decaying willow and alder stumps.  Little is known of the life history but adults occur throughout the year and may be present in large numbers in the spring e.g. we extracted them in numbers from decaying vegetation taken from gravel pit margins in South Herts during May 2012, they are otherwise best sampled by sieving suitable material over a sheet with a pooter at the ready as they can move very quickly when alarmed.

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Adults are small, 3-4mm, and fusiform, suggestive of Tachyporinae or some other Aleocharinae but distinguished by the finely pubescent body, strongly raised lateral borders to the abdomen and diminutive tarsi which are unique among our rove beetles in having three segments. Antennae and body entirely black but for the abdominal apex, maxillary palps variously pale. Legs dark with the tarsi and usually the knees pale.  Head transverse and evenly convex with weakly convex eyes which from above are about as long as the curved and strongly tapering temples, clypeus obliquely impressed behind the antennal insertions and labrum narrow, widely transverse and sinuate anteriorly. Mandibles long and strongly developed, the outer margin sinuate at the base and straight to a curved apical tooth, inner margin with numerous sharp teeth in basal half and three large and sharp teeth towards the apex. Antennae 11-segmented and filiform with all segments elongate. Maxillary palps as long as the four basal antennomeres, apical segment long and gradually broadened to rounded apices. Pronotum transverse, broadest just before the base and rounded to a curved anterior margin, basal margin strongly sinuate before sharply-produced posterior angles. Elytra transverse and weakly curved laterally, basal margin almost straight across the middle and  strongly sinuate before the posterior angle. Basal abdominal tergites strongly transverse with broadly raised borders, the fifth almost quadrate and the sixth long and curved apically, the apex characteristic, with two long appendages. Legs long and slender, all femora visible in normal setting, external margin of tibiae with several longer setae, tarsi short and very narrow, a character that will identify the species with a little experience.