Hydaticus seminiger (De Geer, 1774)
This locally common species occurs throughout most of lowland Europe north to southern Fennoscandia and extending east through Asia Minor and Russia to eastern Siberia, the UK distribution includes the south England west to Somerset, East Anglia and the west midlands, it has been recorded sporadically further north to Glasgow and is widespread in Ireland. Adults occur year-round and are active from early spring until late in the year, they overwinter away from water and return to breed in the spring, typical habitats include margins of small permanent ponds and ditches rich in vegetation in both open and shaded situations; often in wooded or heathland situations and sometimes in small garden ponds where they may be present in numbers. Larvae develop through the spring and summer and new-generation adults occur from late summer. Adults are readily sampled by sweeping marginal vegetation or from baited bottle-traps; they are good fliers and so may occur at light or in new situations through the warmer months.
Adults are easily identified by the size, 13.0-14.5mm and distinctive colouration; black with pale borders to the pronotum and elytra. Head black with the anterior margin and two small maculae between the eyes pale, very finely punctured and microsculptured, antennae and palps pale or darkened apically. Pronotum dark with broad lateral and anterior margins the surface very finely microsculptured and with series of large punctures towards the margins. Elytra black with pale margins from the shoulders to the posterior third, in outline broader and more rounded towards the apex in the female. Legs dark to pale brown. Terminal segment of pro- and meso-tarsi very long, protarsal claws equal, long and strongly curved at the base, metatarsal claws unequal, the outer claw about half the length of the inner claw. Basal segments of male pro-tarsi widely dilated.
Our only other member of the genus, H. transversalis (Pontoppidan, 1763) is smaller, 12.0-13.0mm, and distinct in having a pale transverse band behind the base of the elytra.