Aquatic beetle with an oval, flattened, streamlined shape. The head is sunk into the thorax and the hindlegs are flattened into flippers for swimming; there is a wide variation in size within the species; they are usually dark or black in colour and the entire body has a resplendent sheen. Both the adults and larvae are entirely aquatic, and are common in still, fresh waters such as ponds and lakes.
Classification Water beetles are in family Dytiscidae, order Coleoptera, class Insecta, phylum Arthropoda.
Both adults and larvae are carnivorous. The larvae have a particularly fierce appearance with sickle-shaped mandibles, or jaws. These mandibles have holes at their tips through which the larva secretes digestive enzymes when it catches its prey. So to a great extent digestion is external, the body fluids of its victim being sucked up via the holes and the channels in the mandibles. The pupal stage is terrestrial, being spent under the soil. There are some 4,000 species within this family.
The adult beetles return to the water surface periodically to replenish their supply of air, which is trapped under their wing cases.
In Britain there are 110 species. The common European diving beetle Dytiscus marginalis occurs in Britain. It reaches a length of up to 35 mm/1.5 in, and is dark brown. The head, thorax, and elytra (wing cases) bear a characteristic yellow band along their margins. In the adult male the forelegs bear adhesive pads for holding the female securely during mating.