Hydroporus gyllenhalii Schiødte, 1841

This species has a mostly western and central European distribution, it is locally common from Portugal to Italy, Croatia and Slovakia in the south and reaches north into the UK and the very south of Sweden and Finland, it is absent from Lithuania, Latvia and Estonia although does extend into parts of north western Russia. It is common and often abundant throughout the UK and Ireland, with the exception of certain parts of south and central England, including all the islands north to Shetland and is generally the most common species of acid water habitats. Adults are present year-round, they are active over a long season from February or March until the autumn and peak in abundance from April until June and again in September, they overwinter in marginal habitats out of water but are occasionally active during mild spells in the winter. Most commonly associated with neutral or acid waters and rarely found in chalk or limestone districts, the adults are often common in moorland and woodland pools and may be abundant among sphagnum in open situations, more generally they occur in small bodies of still water, vegetated or otherwise, in a range of open and shaded habitats, they fly well and during the summer may appear in cattle troughs or garden ponds etc., and occasionally come to light. Breeding occurs in spring and early summer and the aquatic larvae develop through late spring and summer to produce adults from late summer that will overwinter. Adults may be swept from marginal situations through most of the year and they may be common in temporary pools in the spring, they usually occur in numbers but often among may superficially-similar species and so will need to be examined carefully.

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​3.4-4.1 mm. Elongate-oval and more-or-less continuous in outline, dorsal surface extensively reddish-brown;  head pale to dark brown with the base and often the anterior margin paler, pronotum dark reddish brown, often with paler lateral and apical margins, elytra coloured as the pronotum or very slightly contrasting paler or darker, dorsal surface finely reticulate and with extremely fine, hardy noticeable, pubescence. Antennae extensively pale at the base, segments 5-11 darkened apically. Head finely and rather sparsely punctured throughout, antennae extensively pale at the base, segments 5-11 darkened apically. Pronotum transverse, broadest across the base and narrowed to projecting anterior angles, lateral margin narrow but distinct, surface evenly though not densely punctured throughout, these vary in size but many are at least as large as those on the elytra. Elytra opaque and without striae or (usually) distinct rows of punctures, evenly and quite strongly punctured throughout, the punctures separated by at least their diameter. Male pro-tarsi not dilated and claws not modified. Aedeagus distinctive, median lobe constricted about the basal third then lanceolate to an acutely-pointed apex.