Hygrotus impressopunctatus (Schaller, 1783)
This very widespread and generally common Holarctic species occurs throughout North America from the tree-line in Canada south to New Mexico, and across the entire Palaearctic region from Portugal through Asia Minor and Russia to northern China and Japan, extending north to the UK and to northern provinces of Fennoscandia; here it is common throughout England and Wales including Anglesey and Man although absent from most of the West Country and upland areas of Wales and Northern England, it is widespread though very local across Ireland and Scotland north to Perth but absent from the western Isles. Adults occur year-round and are active over a long season from very early in the year; they peak in abundance in the spring and again during the summer and are occasionally active during mild spells in the winter. Breeding commences in the spring and larvae develop through the spring and summer to produce new generation adults that will overwinter. Typical habitats are unshaded stagnant ponds and shallow margins of lakes and gravel pits etc that are rich in vegetation but they may also be swept from reed bed margins or ephemeral water-bodies and occasionally occur in brackish pools by the coast. Adults are good fliers and may appear suddenly in well-worked habitats including, as we have found locally, garden ponds. Sweeping among vegetation is the best way to sample the species; adults usually occur in numbers and often among populations of other small dytiscids
4.0-4.5mm. Elongate-oval, flattened and discontinuous in outline; the pronotal base being a little wider than the base of the elytra, substantially brown but with characteristic dark markings. In side view the lateral margins of the pronotum and elytra form an angle of about 45 degrees. Head brown but extensively darker inside the eyes and along the base, antennae pale with the distal segments darkened, at least towards the apex. Pronotum brown with the basal margin variously darkened; usually across the centre and often extending onto the disc but without an isolated central spot, basal punctures moderately strong, those on the disc weaker and of various sizes. Scutellum hidden beneath the produced basal margin of the pronotum. Elytra brown with the base dark (sometimes only narrowly) and various longitudinal darker lines that are often confluent, punctures of various sizes on the basal half; each elytron with 4 longitudinal series of variously confluent punctures that may form more-or-less continuous striae that continue into the apical half. Epipleura with a fine oblique line beneath the humeral angle. Males and some females are shiny and here the elytral punctures are strong and mostly well-separated in the basal two-thirds, becoming weaker towards the apex, some females are dull and finely reticulate between a much finer, denser and more even elytral punctation. Ventral surface shiny black with the prosternum and head brown, metasternum and coxae coarsely punctured and reticulate, abdominal ventrites strongly and densely punctured; the cuticle smooth in the male and reticulate in the female. Front and middle tarsi dilated in both sexes, all claws sub-equal in length; males can be distinguished by the inner claw on the front tarsi being more strongly curved than the outer claw, in females they are similar.