Hi, this is an image of a larva I spotted in a saltmarsh lagoon. They seem to have been aroused by a rise in water level, and appear to breath through a tube at their back end. They are legless, slightly dorso-ventrally flattened, and crawled along the bottom with a maggot like wriggling motion, where the water was shallow. They were between about 3-4 cm in length. Any ideas as to what they are?
In the (larger) Hydrophilidae family, there are about 90 species in Britain. But H. piceus is the best known and largest. it is actually vegetarian, but 'it can inflict a nasty wound on unwary fingers with a sharp spine on the underside' ('Collins Field Guide: Insects of Britain and Northern Europe', 3rd ed., Michael Chinery, 1993, p.145)