Ochthebius dilatatus Stephens, 1829
Among the most common and widespread of the UK members of the genus, this species is common throughout England and Wales including the islands and is very frequently recorded in coastal districts, it is mostly eastern in Scotland but is also present in the Hebrides and Orkney, and it is widespread though largely coastal across Ireland. The continental distribution is patchy; it occurs from Portugal to Denmark and southern Sweden but is absent from most of the Baltic countries, it is present throughout the Mediterranean region including North Africa and most of the islands, reaching the Black Sea, and also recorded from brackish margins of the Caspian Sea. This is a stagnant water species which inhabits moss and algae in shallow marginal habitats; it usually occurs in natural ponds or around lake or reservoir margins, it is often common in brackish pools and salt marsh margins, it sometimes appears among seaweed along the strand line and may occasionally appear in temporary habitats such as flooded tyre ruts in woodlands during the spring. Adults occur throughout the year, peaking in late spring and late summer, and regularly appear among moss and litter samples taken from marginal situations through the winter, they are active over a long season from early spring and will appear when sweeping vegetation in shallow water, during the summer when pond margins dry out they may be found in numbers under stones or among litter and on hot sunny days in late spring and early summer they sometimes swarm in large numbers over wet soil. Adults are generalist feeders, they consume algae and protozoa etc. and spend much of their time submerged on the substrate or among algae in shallow water while the larvae are at best only semi-aquatic and dwell mostly among litter and moss near on wet soil near the water.
1.8-2.2mm. Entirely shiny black with a faint metallic bronze reflection, legs dark brown to reddish brown, antennae dark with the scape and basal funicular segments pale. Head broad, roughly sculptured and finely punctured, with large asymmetric and very convex eyes, and smoothly curved or truncate anterior margin to the labrum. Palps dark brown, the penultimate segment expanded and the terminal segment small, narrow and pointed. Antennae 9-segmented; the scale long and gradually broadened to the apex, second segment large and globular, third and fourth tiny and 5-9 forming an elongate pubescent club. Pronotum widely transverse but distinctly narrower than the elytra, the posterior angles deeply excised and covered with a translucent membrane, lateral margins smooth or extremely finely crenulate and anterior angles obtuse and rounded, surface evenly and rather finely punctured; with a median longitudinal furrow and two variable but usually well-defined impressions either side and a broad but much less well-defined impression towards the margins in the apical half. Elytra oval and convex, at least one third longer than wide, with ten rather strongly punctured striae which become weaker towards the apex and a scutellary striole consisting of at least five punctures, all of which contain a short pale seta, interstices smooth and flat or weakly convex although on the disc, or at least below the scutellum, they are often transversely wrinkled. Legs long and slender, margins of all tibiae with fine spines, tarsi 5-segmented but appearing 4-segmented due to the tiny basal segment, the terminal segment in each case long and weakly curved, claws smooth and without a basal tooth. Males can be distinguished by their slightly dilated front tarsi.