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.