Limnebius truncatellus (Thunberg, 1794)
This generally common species occurs throughout Europe, except for the extreme south, north to the Arctic Circle in Scandinavia and across Siberia, from lowlands to subalpine altitudes in a wide range of wetland situations. Here it is common and often abundant throughout the UK to the far north of Scotland including Orkney and Shetland. Typical habitats include the margins of still and slow-moving water, from weed-choked stagnant ponds to lake and reservoir margins, heath and moorland ponds, sphagnum bogs, dune slack ponds and slow moving rivers, and during the spring and summer they may occur in abundance in temporary situations such as flooded tyre-ruts etc. in wooded areas. Adults occur year-round, they are active during mild winter spells and appear early in the spring depending upon the season, are readily sampled by sieving among marginal vegetation or by flooding substrate and they usually occur among other species e.g. helophorids, hydrophilids and hydraenids including other Limnebius and so a decent sample should be taken for close analysis. They may remain submerged on substrate or vegetation for extended periods as they breath via a ventral plastron; they periodically come to the surface and penetrate the water using their antennae, this connects the plastron with the atmosphere, and they then contract and expand the abdomen to change the air. Breeding occurs in the spring when eggs are laid out of water among damp soil or algae etc. Larvae emerge after a week or so and live among the marginal substrate feeding on algae and other small organisms, they are fully developed within about 2 months and bury into soft soil to pupate in a cell near the water edge. New generation adults appear in late spring and early summer; by July they are abundant and may be found in just about any marginal or temporary wetland situation, and these will go on to overwinter under stones etc. or among moss or tussocks near the water edge. Adults may thus occur among winter extraction samples from suitable habitats.
Limnebius are distinguished among our fauna by the form of the palps and the smoothly elongate-oval body. Males are larger, 2.4-2.7mm, than females, 1.9-2.2mm. Body entirely dark or with the margins of the pronotum and elytra gradually, and sometimes only indistinctly, pale, appendages variously but usually extensively pale. Dorsal surface very finely pubescent, finely but distinctly punctured (X15) and, except for the pronotal disc, finely shagreened. Head smoothly convex, broadest behind the eyes then strongly constricted to a short neck, eyes large and continuous with the outline, labrum usually only narrowly visible from above; strongly transverse and curved anteriorly. Antennae 9-segmented, with a pubescent (hydrophobic) 5-segmented club, palps at least twice as long as antennae, the terminal segment as long as the penultimate. Pronotum smoothly convex and widest anterior to obtuse posterior angles, disc smooth and shiny or only indistinctly shagreened between punctures, margins distinctly shagreened. Elytra smoothly convex; without striae or sculpture but finely and distinctly punctured; broadest behind shoulders and narrowed to separately rounded or truncate apical margins which leave at least the last abdominal tergite exposed. Male with 6 abdominal sternites, female with 7. Male legs modified; pro- and meso-tibiae incurved and dilated apically, meta-tibiae strongly emarginate in basal third, pro- and meso-tarsi with ventral sucker-hairs. Tarsi 5-segmented but appear 4-segmented due to the tiny basal segment.