Hypnoidus riparius (Fabricius, 1792)
This Holarctic species is widespread in Central and Northern Europe, where it extends to the north of Scandinavia, and is noted as occurring up to the alpine and mountain zones. It also occurs in Iceland. It is locally common and in some temperate areas has occurred in large numbers and the larvae have become crop pests. In the U.K. it is widespread to the far north including Orkney; most records are from Wales, the midlands and the north of England where it is locally common, and it occurs through the Scottish Highlands, elsewhere it is local, and there lowland records scattered across the south of England. The adults occur from May to August and are semi-aquatic, occurring among sand or gravel and detritus, marginal vegetation or under stones or debris beside slow-moving streams, lakes and ponds etc. During warm weather they can be swept from suitable marginal vegetation. The larvae inhabit sparsely vegetated and open, often sandy or gravelly soil in wetland situations, often in habitats that are subject to periodic flooding. They develop through the summer and pupate in late summer or early autumn, the adults eclose in the autumn and overwinter in the soil or under stones etc.
5.5-7.0mm. A small and rather nondescript elaterid; broadly oval in outline with separately rounded elytra and pronotum, the body is shiny black with a metallic bronze reflection, the appendages pale; legs pale brown generally with darker femora, and the antennae and sometimes the epipleura dark brown. The dorsal surface is clothed with fine recumbent grey or yellowish pubescence. It is distinguished from other European members of the genus by the presence of fine reticulate microsculpture between the pronotal punctation and the smoothly rounded aedeagal parameres which lack an apical tooth. The head is finely punctured and flat with prominent mandibles which are directed forward or slightly angled down. The antennae are serrate from the fourth segment and the second segment is as long as, or nearly as long as, the fourth. Pronotum transverse and very convex, finely beaded laterally and with the median furrow short and well-impressed. The surface is finely and diffusely punctured and the hind angles produced and sharp with a distinct ridge along the dorsal surface. Hind margin bisinuate. Scutellum large and flat. The elytra have well-impressed and impunctate striae, the interstices are convex towards the base and apex and are finely punctured. The tarsomeres lack lobes and the claws lack setae at the base.