Carpelimus rivularis (Motschulsky, 1860)
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POLYPHAGA Emery, 1886
STAPHYLINOIDEA Latreille, 1802
THINOBIINI Sahlberg, J., 1876
Carpelimus Leach in Samouelle, 1819
A Palaearctic-wide species present throughout Europe north to the UK, Denmark and the Baltic countries where it reaches into southern provinces of Fennoscandia, also recorded from many of the Mediterranean islands, North Africa and the Canaries. There are several records from Canada, and it is likely to have established there since at least the early 20th century but having until recently been confused with a closely similar species. Generally common and often abundant throughout much of the European range and usually among the most frequently recorded members of the genus. In the UK it is generally common throughout England and Wales and less so further north into Southern Scotland and across the north of Ireland. The species occurs in all kinds of wetlands, including salt marshes and coastal flats, they generally occur in or on surface of wet soil, often among dense vegetation near the waterline, and usually in numbers. Adults are present year-round; they overwinter among algae or tussocks although they may remain active in all but the coldest weather, and peak in abundance during late spring. Little is known of the biology but adults are thought to feed on algae (this has been proven for several members of the genus), and larvae probably develop through the spring and summer in the same habitats as the adults. The adults are easily found by sieving or extracting likely samples, flooding or compressing damp substrate, or by pitfall-trapping, they usually occur in large numbers but specimens will need to be taken for careful examination as two or more species often occur together, they also fly well, occasionally swarming on warm evenings and sometimes coming to light traps.
2.5-3.2 mm. Elongate and rather flat, entirely shiny black to very dark brown, forebody with dense pale pubescence, elytra with shorter and more erect golden pubescence, antennae dark, usually with paler basal segments, legs dark with paler tarsi. Head with large convex eyes and angled temples that are at least as long as half the eye diameter and that project laterally to about the outer margin of the eyes. Vertex and frons with finely and discretely punctured. Terminal maxillary palpomere diminutive and needle-like. Antennae inserted laterally, the insertions not visible from above, 11-segmented and only slightly widened towards the apex, segment 5 elongate and segment 10 quadrate or slightly elongate. Pronotum transverse, broadest in front of the middle and narrowed to obtuse angles, basal and apical margins weakly curved, surface with a longitudinal arcuate depression either side of the disc and an obscure transverse impression in front of the base, punctation of two types; coarser and denser punctures from the lateral margins which extend to about a quarter of the distance to the centre, and finer and discrete punctures on the disc. Elytra elongate and distinctly broader than the pronotum, parallel-sided or slightly dilated from rounded shoulders to an almost continuous apical margin, surface uneven, often with obscure longitudinal depressions from the base, and finely and discretely punctured throughout. Abdomen usually dilated from the base, tergites impressed across the base and raised laterally, surface with cellular microsculpture and punctured throughout, more finely so towards the apices. Tibiae without spines, tarsi 3-segmented. Specimens can be identified from the above characters if reference material is available; the widespread C. similis Smetana, 1967 is closely similar with the same type of pronotal punctation, but males of the present species are readily identified from the form of the aedeagus in which the callipers are reduced and there is dark and bifurcated hook-like structure obvious among the internal sclerites.