Hydroporus memnonius Nicolai, 1822
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ADEPHAGA Clairville, 1806
HYDROPORINI Aubé, 1836
This species is widely distributed throughout most of Europe but is more common in central and northern regions, it occurs from lowlands to about 2000 m and extends north to Faroe, the UK and above the Arctic Circle in Fennoscandia, it occurs on most of the Mediterranean islands and is widespread across North Africa, Asia Minor and east to Turkmenistan, it is sometimes quoted as occurring in North America but this seems to be in error. It is generally common and often abundant throughout the UK, occurring on all the islands north to Shetland. Two distinct forms are known and both are widespread across Europe but in the UK are more-or-less partitioned geographically; females occur in shiny and matt forms, the latter often referred to as var. castaneus Aubé due to its generally chestnut-brown colouration, while males are always shiny although they differ in tarsal morphology with var. castaneus males having suckers on the second front and middle tarsomeres while these are lacking in normal males, var. castaneus is predominantly southern in distribution while the normal form is mostly northern, they overlap in northern England and southern Scotland and the matt form seems to be undergoing a recent northward expansion which is not due to climate change, although the normal form has been shown to more cold-tolerant, but rather to sexual competition and matt females are generally displacing normal ones. Adults occur year-round; they overwinter among marginal substrate and are active over a long season from February or March until late in the autumn, peaking in abundance between May and July. Typical habitats are small water bodies rich in moss and organic detritus, often in shaded situations or woodland but they fly well and during the summer may be quick to colonize new habitats such as garden ponds and often occur in cattle troughs and water butts etc. During the spring they may occur in temporary habitats such as flooded tyre-ruts in woodland or on heaths, and they occasionally occur in huge numbers; we found many thousands of specimens in a small muddy pond formed beneath a fallen tree on Bricket Wood Common during June 2012, the pond was in heavy shade, no other dytiscids were present and the sexes were present in roughly equal numbers. Breeding occurs over a long season during the spring and summer, teneral adults are present from early spring into the autumn and larvae are known to overwinter.
3.8-4.3mm. An elongate, more-or-less parallel-sided species which is continuous in outline, colour varies from almost black to chestnut-brown although the forebody is often darker than the elytra, ventral surface black with the prosternum and epipleura dark brown. Most specimens are distinctly shiny, with only a fine reticulate microsculpture but ‘dull’ females vary, their microsculpture usually deeply impressed. Head predominantly black, usually pale anteriorly and narrowly across the base but sometimes extensively pale, surface evenly convex and very finely punctured, antennae variable but usually brown with the apical half of segments 5-10 darker. Pronotum transverse and strongly bordered laterally, broadest near the base and curved laterally to projecting anterior angles, surface evenly convex with scattered larger punctures, extensively dark with variable pale lateral and apical borders. Scutellum not visible. Elytra opaque; dark to pale brown although sometimes contrastingly paler beneath the shoulders and around the apex, rather coarsely punctured throughout (finely so in matt females) and with two variable but obvious longitudinal rows of larger punctures. Males have equal front tarsal claws but may be distinguished by the more strongly dilated basal middle and front tarsal segments.