Dytiscus marginalis Linnaeus, 1758
Great Diving Beetle
This very widespread and generally common species occurs throughout Europe to the far north, and east through Asia to Japan; in the east it is represented by the subspecies czerskii Zaitsev, 1953. It is widespread across the U.K. to the far north of Scotland, where it occurs above 500m, including the Western Isles but not Orkney or Shetland. They occur in most still or slow-moving aquatic habitats and are common or even abundant where found, they will colonize small water bodies e.g. garden ponds or even cattle troughs but generally prefer habitats on soft and silty or muddy substrates with plenty of vegetation although they seem to be less common in chalk or limestone areas. Adults occur year round; they spend the winter hibernating in the substrate at the bottom of ponds etc. and emerge early in the year, late winter or early spring depending on the season, and disperse by flying at night, at this time new environments are quickly colonized. Both the larvae and the adults are voracious predators, feeding on a wide range of aquatic life including newts and fish etc. and will also consume carrion, the young larvae are also cannibalistic at high densities. The adults will exude a foul smelling liquid from the abdomen when alarmed or threatened and this will generally deter predators. Eggs are laid in the spring when the females use the ovipositor to cut into the stems of aquatic plants and insert a single egg at a time, each female will work her way along a stem inserting eggs at short intervals so that, after a few days, there may be a sudden swarm of the small and transparent larvae. Despite being hidden in the plant stems the eggs are sometimes parasitized by tiny chalcoid wasps. Larvae begin hunting as soon as they emerge from the eggs, they obtain air by hanging upside down and protruding the abdomen through the water surface and because they are buoyant at this stage they spend much of their time thus suspended, snatching passing prey. Their large and curved mandibles are hollow and, after prey is caught, used to inject poisonous digestive juices which quickly disable it and then begin to liquefy the body contents, then after the contents have been sucked out the prey is discarded. They grow rapidly, increasing greatly with each moult, and the final instar is large and heavy so at this stage they live in shallow marginal
water where they can obtain air without having to swim to the surface. By late summer they are fully grown, reaching some 60mm in length, and leave the water to dig small holes in the damp marginal soil where they will pupate. This stage is also rapid and the adults eclose after only a few weeks but remain buried; newly emerged they are soft and white but over a few days they harden and develop the adult colouration. They are then active until late in the autumn when they will hibernate among the substrate at the bottom of ponds etc.
A large and very distinctive species; 26-36mm elongate-oval and continuous in outline, males vary from shiny black to brown and usually to some extent green, often strikingly so when submerged. Females are dull brown. The head is dark with the clypeus and a V-shaped mark on the frons pale, the inner margin of the eye is also sometimes narrowly pale. Pronotum dark with broad pale borders. Elytra with pale borders and sometimes an oblique subapical pale mark. Appendages pale. Ventral surface mostly pale; the metasternum, meta-coxae and the apical margins of the abdominal sternites are darker, sometimes almost black. The metacoxal process is distinctive with the apices acute and blunt, not acuminate, and the inner margin straight or weakly convex. Both smooth and sulcate females occur in northern Europe but only sulcate specimens are found in the U.K. Males can always be assigned with confidence by the dilated pro-tarsi; more subtly the males have long swimming hairs on both sides of the meta-tibiae whereas in the female they are only present on the outer edge.