Hydroporus palustris (Linnaeus, 1761
A common and often very common water beetle occurring throughout mainland UK and the islands: Isle of Wight, Lundy, Anglesey, Man, the Western Scottish islands, Orkney and Shetland although records are a little more scattered and less dense through Wales, the west country, around Perthshire and the northern highlands. Balfour-Browne considered this due partly to collectors failing to record such a common species but also notes that 'it is not usually a peat inhabiting species, its place being taken by (Hydroporus) incognitus.' Usual habitats are diverse and include all types of still and slow running freshwater. Nilssen and Holmen (1995) state 'also above the tree line' and 'in the north, a marked preference for lakes and large bodies of running water.' Our local experiences are similarly diverse; we have records from weed-choked stagnant ponds as well as from slow running drainage ditches where clear water runs over exposed sediment and gravel below Cassiobury park, from among the reedbeds and bordering open water at Radlett road and among aquatic vegetation along the river Colne from Watford south to Cowley. During the 1980's the species was common around the margins of large, recently excavated gravel pits largely devoid of vegetation between Denham and Harefield, and in a small and stagnant garden pond at Ruislip. Adults appear locally during June or July and soon become abundant, remaining so through the summer and into the autumn. Sweeping among and around the margins of aquatic vegetation or sieving samples of bottom mud etc will often produce the adults and they occasionally turn up at light. Balfour-Browne records 'soft, newly emerged imagines in July', and Nilssen and Holmen state 'overwintering adults and summer larvae.'

So common and widespread a species as palustris will sooner rather than later be encountered in the pond net and probably in some numbers. It is generally unmistakeable and readily identified and so should be used as a model with which to become familiar with the form of some of the structures used routinely to identify Hydroporus species. The following description is a basic account of the upperside morphology and following this are descriptions of some older, more subtle, characters.

3.3-4.0mm. Elongate oval; the elytral outline is smoothly curved from base to apex, that of the pronotum varies a little and in some specimens is less curved than in others but the effect of the two separate curves is to produce a distinct constriction at the pronotal/elytral junction. Entire upper surface with well impressed microsculpture which appears isodiametric at X20. Pronotum and elytra with long, quite dense recumbent pubescence. Head pale to dark orange with a dark triangular mark inside eyes (in our specimens these marks do not meet at the centre) and a dark mark behind the eyes which extends along the base. With an ill-defined and shallow oblique furrow inside each eye extending to lateral margin of clypeus. Puncturation sparse and fine. Eyes continuous with outline of head, emarginate along front margin. Antennal insertions under clypeus, not visible from above. Basal segments of antennae testaceous, from 4 or 5 infuscated towards apex, apical segments extensively darkened. Pronotum black or piceous with lateral broadly and characteristically yellow; black colouration extends along front and hind margins to, or almost to, sides, pale colouration extends towards disc medially. Transverse, broadest a little behind middle (varies), front angles projecting, hind angles slightly obtuse, lateral margin finely edged. Puncturation wide and shallow, dense behind front and hind margins, much less so as disc. Scutellum not visible; hind margins of pronotum projecting back medially. Elytra black or very dark brown with characteristic orange/yellow pattern as shown, this varies in extent and the elytra may be extensively dark but the pattern will be seen if an elytra is lifted under strong light. Microreticulation as on pronotum, punctures shallow and wide, as on pronotum or a little wider, density varies but often as on pronotum. Without, or with only very ill-defined longitudinal series of punctures, those from humerus usually discernable when viewed from apex. Epipleurs broad from humerus to metasternum then strongly narrowed to apex. Legs testaceus or dark testaceus. Pro and mesotarsi 4 segmented; 1-3 dilated in both sexes but a little more so in male. 3 deeply bilobed. Claws relatively large and easily examined at X20; inner protarsal claw in male a little broader than the outer and appears shorter.

Prosternal process weakly convex, without transverse lines across the base but with a step just before the anterior margin of the procoxae.

Aedeagus narrow and pointed with sides evenly tapering. Inside edge in lateral aspect almost straight from basal curve to apex.

Description from 7 Watford specimens at X20