Hydroporus Clairville, 1806

The species are small to medium sized water beetles, <1.5 mm to a little more than 7 mm long (UK species from 1.9-5.5 mm), very typical of the subfamily with the scutellum not visible, the front and middle tarsi apparently 4-segmented ( the fourth segment is small and usually hidden within the lobes of the third) and the hind tarsal claws are equal in length or nearly so. They are elongate oval and only moderately broad, most species being distinctly elongate, the outline varies from continuously to separately rounded, many are rather parallel-sided and none approach the broad form seen in genera such as Hydrovatus Motschulsky, 1853, Hyphydrus Illiger, 1802 and some Hygrotus Stephens, 1828.   Most are drab brown or reddish-brown to black, often with the forebody darker, or at least contrasting with, the elytra although many have paler pronotal or elytral borders and some are maculate; many species vary in colour, either regionally or in response to environmental conditions, and even maculate species may have entirely dark forms. Antennal colour can be a good guide to various groups of species. The ventral surface is mainly dark grey or black but even in substantially pale species the metasternum and basal ventrites are usually dark. Dorsal microsculpture varies widely both in intensity and form and sometimes between the sexes, being often stronger in females, it may be entirely absent, present only in places or extend over the entire beetle, it can be useful for identification but may vary between the sexes or even between specimens of the same species, and in a few cases e.g. females of H. memnonius Nicolai, 1822, both smooth and strongly microsculptured forms occur. Dorsal pubescence also varies widely; most species are largely glabrous or only sparsely pubescent but a few are densely pubescent across the pronotum and elytra. Because of the relatively large number of species; about 200 in total of which about 80 occur in Europe and 28 in the UK, and because many vary in colour and microsculpture etc they can be very difficult to identify and dissection is often needed to be sure, but the genus is distinct and will soon become familiar. They are rather ordinary looking dytiscids, among our UK species very few have distinctive patterns and this will immediately distinguish them from many of our other smaller water beetle genera, they may otherwise be distinguished by the following combination of characters: anterior clypeal margin simple, pronotum and elytra without longitudinal impressions, elytral epipleura without an oblique impressed line or crease towards the base,  metacoxal process truncate or sinuate apically, sometimes weakly produced medially, and the ventral surface of the hind femora with a single row of punctures; never extensively punctured. The form of the prosternal process is sometimes useful in diagnostic work; it is narrow and long, contacting the metasternum, and with a transverse basal ridge and/or a distinct border towards the apex, in many there is a distinct ‘file’ composed of fine transverse grooves before the basal impression. Despite the close similarity of several species and the generally drab nature of many, the typical colour – even when it is black – coupled with the size and, especially, the outline shape, will soon make many distinctive even in the field. Most specimens can be assigned from gross morphology but for critical work the male genitalia will often need to be studied and in some cases this is the only reliable guide, so dissection will be necessary but this is straightforward with a little practice and the extracted parts can be glued to the specimen card for future reference; all of these structures are figured in several very good modern guides and many can also be found online. Males often have the basal meso- and pro-tarsomeres broadened and equipped ventrally with enlarged adhesive setae but sometimes these features do not differ between the sexes, and males may sometimes be recognised by a tooth or lobe on one or both of the pro-tarsal claws.