Elmidae

Generally found amongst stones and gravel in the riffles of fast-flowing rivers and streams, there are 12 British species.  All are 1-5mm long and walk on the substrate rather than swimming: indeed, that have to hold on to the substrate due to their positive buoyancy, and if disturbed will float to the surface.  Adults are dark brown to black and oval to elongate, with striate elytra, large tarsal claws and long threadlike antennae.  Away from riffles, Elmidae may be found amongst stones on lake shorelines, slow-flowing sections of rivers and drains, and on submerged logs.