Hydroporus planus (Fabricius, 1782)
This is a generally common species throughout lowland Europe and North Africa, including the Mediterranean islands, extending north into northern Fennoscandia and east through Russia and the Near East to Western Siberia, here it is generally abundant throughout lowland UK and is present on most of the islands north to the Shetlands. Adults occur year-round in a wide variety of wetland habitats; mostly in well-vegetated standing water on clay or silty substrates but occasionally in slow-moving stretches of rivers and ditches, but during spring and early summer they may occur in garden ponds and cattle troughs etc, often in abundance and usually along with other small dytiscids; we have sampled them from such diverse habitats as local reed beds, watercress beds and gravel pits, and from peat cuttings in North Somerset. This is sometimes the predominant small dytiscid species in permanent marshes and may also occur in numbers in brackish water; adults are readily sampled by sweeping through marginal vegetation, although in warm weather they may be very active and take flight from the net or sample tray. In most situations it should be among the first of the small water beetles to be collected. Reproduction is thought to continue through the spring and summer with eggs as well as adults overwintering; adults usually become active early in the year and may also occur during mild spells in the winter.
3.8-4.8mm. A rather nondescript broadly oval, rather shiny and drab-coloured species, the dorsal surface is finely and sometimes densely pubescent and the pale elytral base, at least about the humeri, contrasts with the black pronotum. The pronotal disc and elytral base (at least) are smooth and lack reticulation and the apical abdominal sternite has a fine isodiametric reticulation, the prosternal process is smooth across the base (i.e. without a transverse impression), strongly narrowed between the coxae and lanceolate apically, and the metacoxal process is shiny and finely pubescent. The aedeagus is smoothly and only weakly narrowed about the centre and the apex is broad and almost truncate. Head substantially black, usually with the anterior and basal (often hidden within the thorax) margins pale, base of antennae pale, segments 5-11 dark, at least towards the apex. Pronotum continuous in outline with the elytra, entirely black but for very narrow paler lateral margins, reticulation usually obvious around the lateral and apical margins. Elytra dark brown; usually pale across the base and laterally but this may be confined to the humerus, in any case there is an abrupt contrast with the black pronotum, surface densely and very finely punctured and pubescent and distinctly reticulate towards the apex. Ventral surface black, contrasting with the mostly pale legs and elytral epipleura. Legs variable from pale to very dark brown. Males may be distinguished by the slightly dilated basal pro-tarsomeres and more strongly curved claws.