Hydraena riparia Kugelann, 1794
This is a generally common species of lowland and low mountain altitudes throughout Europe from Portugal to the Caucasus, north to the Arctic Circle in Scandinavia, and east through Russia to the far east of Asia although some of these far eastern populations have proved to belong to other species in what is called the ‘Hydraena riparia complex’. In the UK it is generally our most common species and is widespread and often abundant to the far north of Scotland although seemingly absent from the Western Scottish Isles, Orkney and Shetland. Typical habitats are marginal wetland situations beside still and slow-moving water, often around stagnant pools and lake margins or among detritus beside reed beds. Adults occur year-round; we have extracted them from samples of marginal detritus from local reed beds throughout the year; they become active in the spring when numbers in swept samples of aquatic vegetation suddenly increase and they may occur in large numbers among waterline detritus etc. They are partly aquatic and may spend long periods among submerged vegetation or substrate, and in the spring may suddenly appear in large numbers among wet marginal soil or matted vegetation, they are generalist feeders in both aquatic and marginal situations whereas the larvae develop in wet soil etc. near the water edge. Mating occurs in the spring and eggs are laid among marginal soil where the larvae will develop and pupate. Adult numbers peak in early summer and they remain active into the autumn, they will usually be found among numbers of other marginal species e.g. hydrophilids, helophorids and other Hydraenids including other species of Hydraena.
Distinct from superficially similar species by the form of the pronotum (Helophorus, Hydrophilidae) and the 5-segmented antennal club (Hydrochus). Among our Hydraena species distinguished by the overall size and shape, the numerous puncture rows at the base of the elytra, and the form of the pronotum. 2.0-2.5mm. Elongate and rather parallel-sided, discontinuous in outline and entirely dark brown or with the head darker and/or the elytral apex diffusely paler. Head produced anterior to strongly convex eyes, vertex strongly punctured, clypeus much less strongly so, labrum curved and finely
crenulate anteriorly and with a deep median notch. Without obvious structure but usually with a shallow impression beside each eye. Pronotum broadly hexagonal; widest about the middle and equally narrowed to distinct posterior and anterior angles, lateral margins denticulate, surface weakly convex and strongly punctured, about the same as the head, and with a variable shallow and wide impression parallel to the lateral margins. Elytra elongate-oval; about 2/3 longer than wide, with distinct convex shoulders and evenly curved to separately rounded apices, the sutural margin produced into a fine apical tooth; with 15 finely and densely punctured striae, 7-10 at the base between the suture and the humeral callus, interstices very narrow; generally narrower than the strial punctures on the disc. Lateral margins weakly explanate from the basal third to apex. Appendages pale brown. Antennae 9-segmented, with a 5-segmented club. Maxillary palpi much longer than the antennae; terminal segment longer than penultimate, in the male with a curved tooth at the apical third. Legs long and slender, inner margin of mesotibiae finely crenulate in apical half, otherwise unmodified. Tarsi 5-segmented but usually appearing 4-segmented due to the tiny basal segment.