Hydroporus palustris (Linnaeus, 1761)
This species occurs throughout Europe from Spain to Greece and Russia and north to the UK, Faroe Islands and beyond the Arctic Circle in Fennoscandia. To the east it extends through Asia Minor and Russia to Siberia and Mongolia, It is absent from North Africa and the Atlantic islands and across southern Europe it tends to be local and sporadic although it is often common at higher altitudes and it reaches the alpine zone in most mountainous areas while in the north it is generally common and often abundant, it is common throughout the UK and occurs on all the islands north to Shetland. Adults overwinter among soil etc. close to aquatic habitats and become active early in the year, they peak in abundance during spring and summer and are generally common into the autumn. Typical habitats are small stagnant ponds and shallow lake margins on muddy or silty substrates and with plenty of aquatic vegetation although they also occur, much less commonly, in running water and reservoirs. More generally they may be found in a wide range of situations from shaded pools in woodland to exposed moorland ponds and sphagnum bogs, reed beds and drainage ditches and even small garden ponds and temporal woodland puddles, they fly well and disperse on warm spring and summer evenings and so might suddenly appear in new situations. Breeding occurs in early spring and eggs are laid singly or in small groups among the substrate or on aquatic stems etc. First instar larvae appear from April, they are predaceous and develop rapidly, passing through three instars and leaving the water to pupate among marginal moss or in a subterranean cell from late May or June. Fresh adults return to the water where they will spend most of their time until they leave to overwinter; during the summer their smaller habitats may dry out and they will either disperse or move into the soil or under stones or debris until the water returns. Adults are easily sampled by sweeping among marginal vegetation and they occasionally appear at light.
Hydroporus palustris 1
Hydroporus palustris 2
Hydroporus palustris 3
3.3-4.0 mm. Elongate-oval and distinctly constricted between the pronotum and elytra, head pale to dark brown with variable darker markings along the base and inside the eyes, pronotum dark with variable but usually broad paler lateral margins, elytra variable but usually characteristically bicoloured pale brown and black, entire dorsal surface with distinct microsculpture (X20), pronotum and elytra with fine pale pubescence. Head obliquely depressed from the eyes, otherwise smoothly convex and finely punctured, eyes emarginate anteriorly and continuous with the outline of the head, antennae inserted under the clypeus, substantially dark with several basal segments pale. Pronotum transverse, broadest across near the base and narrowed to projecting anterior angles, apical margin curved, and basal margin produced medially, surface more strongly and densely punctured across the base and behind the anterior margin. Prosternal process smooth across the base, metasternal process obliquely truncate. Scutellum not visible. Elytra broadest at or behind the middle, colour varies from almost completely dark to extensively pale but usually dark with the pale base and lateral margins forming a distinct pattern, epipleura broad from the base to the metasternum then strongly narrowed to the apex, in lateral view the keel curving up towards the base and forming an obtuse angle with the pronotal margin. Legs entirely pale. Tarsi apparently 4-4-5 because of the tiny fourth segment, basal segments of the front and middle tarsi dilated in both sexes but more so in the male. Front claws unequal in the male; the inner claw a little broader and shorter than the outer claw.