Hygrobiidae (=Paelobiidae)
With a single species Hygrobia hermanni (Fabricius, 1775), the Screech Beetle, considered to be common across southern England and the midlands becoming much rarer further north. Adults may be found around the margin of still water, usually where the bottom is soft silt or mud and often where the water is ammoniacal from the attention of cattle. It's inclusion on the Watford list is based on a single specimen from Radlett road lakes during June 2006 taken from silt among emergent vegetation. During the 1980's the species was common along the Colne valley south of Watford, especially so from the barren edges of reservoirs, usually occurring in groups of up to twenty during the warm summer months. As soon as vegetation began to shade the water the beetles vanished. Recent (2004) sampling of its former habitats failed to find the species. Although we cannot comment on whether it is declining we cannot but be surprised at its scarcity.
Adults occur from March to October. Eggs are laid in rows on the surface of aquatic vegetation from late March until July and, during the warmest part of the season, take around nine days to hatch. The crustacean like larvae are entirely aquatic, breathing by gills and living among the bottom ouze and detritus. It is said to feed predominantly on tubifex worms. The length of the three larval stages varies widely but in general adults emerge between nine and fifteen weeks after egg laying. Pupation occurs out of the water in an earthen cell, the stage lasting about sixteen days but the adult remains within the cell for a week or so after emergence. Adults disappear during the autumn and are presumed to hibernate buried in submerged mud.(Balfour-Browne)
Adults have commonly been referred to as the Screech Beetle from their habit of stridulating loudly when alarmed. The sound is produced when the sharp edge of the 7th abdominal tergite is rubbed against a subapical median file on the elytral undersurface.

The Hygrobiidae are characterised by the presence of an antecoxal sclerite, a small transverse scleroticised plate lying between the posterior edge of the metasternum and the post coxal processes.
In practical terms distinguished from other water beetles by the structure of the head; produced in front of the eyes and narrower than the anterior margin of the pronotum, and the elytra lacking striae.

8.5-10mm. Brown, appendages lighter. Head laterally, anterior and posterior margins of pronotum and majority of elytra dark grey or black. Eyes convex and prominent. Entire upper surface punctate; head and thorax finely so, elytra very coarsely so posteriorly from a weak transverse depression in front of middle. Undersurface very convex.