This family is represented by a single species in the British Isles, the screech beetle Hygrobia hermanni (Fabricius).  A large (9mm) yellow and black water beetle, it is found in ponds and ditches, often in the soft mud where both adults and larvae are predatory.  It can stridulate loudly when disturbed, hence the common name.

Formerly part of Dytiscidae, this group has only two representatives in the British Isles.  Both species are found mainly in stagnant water in association with floating rafts of vegetation, often burrowing through mud at the bottom of the pond.  Unusually, members of this family pupate in cocoons attached to the roots of water plants.  The predatory adults can be recognised by the presence of a ‘noterid platform’, a plate between the second and third pairs of legs.

A family of 19 small (3-5mm) yellowish water beetles of slow-moving or static lowland waterbodies.  Adults are roughly elliptical, partly carnivorous, and swim clumsily by using the legs on each side alternately.  When not swimming, they can be distinguished from other water beetles by their large, flat hind coxal plates, which are used as air reservoirs.  Larvae feed on algae and duckweeds, and consequently have specialised mouthparts which distinguish them from the carnivorous larvae of other groups.

This family, with 12 British species in Britain, are most familiar as groups of small dark elliptical beetles swimming in circles on the open water of ponds and streams.  Scarcer species live amongst emergent vegetation, rather than circling openly by day.  All are carnivorous, feeding on animals caught in the surface film.  Larvae breathe through gills, rather than from the surface, which may explain the group’s predilection for deeper water bodies.

A single species in Britain, Sphaerius acaroides Waltl.  Approximately 0.6mm long, hemispherical and black, it looks very similar to a mite but walks like a beetle.  Found mainly in moss adjoining small bodies of water, mainly in Dorset but records are relatively widespread.  Historically regarded as scarce, but its small size and cryptic habits mean it is likely to be overlooked.