Life history[ edit ] Adult beetles and their larvae are aquatic but the pupae spend their life in the ground. Females lay eggs inside the tissue of aquatic plants such as reeds. The eggs hatch in about three weeks. The larvae known as "water tigers" are elongate with a round and flat head and strong mandibles. They are predatory and their mandible have grooves on their inner edge through which they are able to suck the body fluids of their prey.
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It is widespread across the U. 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. Adults occur year round; they spend the winter hibernating in the substrate at the bottom of ponds etc. Both the larvae and the adults are voracious predators, feeding on a wide range of aquatic life including newts and fish etc. 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. 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. 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.
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.
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Male front leg from underside Adult Larva The great diving beetle Dytiscus marginalis is an aquatic diving beetle native to Europe and northern Asia , and is particularly common in England. The great diving beetle, true to its name, is a rather large insect. The larvae can grow up to 60 millimetres 2. These beetles live in fresh water , either still or slow-running, and seem to prefer water with vegetation.
Dytiscus marginalis - the Great diving beetle Click on one of the pictures for a larger picture. Dytiscus marginalis Great diving beetle, male On the left a picture of a Great diving beetle, resting under water. The respiration spot at the hind tip of the abdomen is closed because the beetle has pressed that tip firmly to the elytra, the wing cases that cover the back. The border of the air supply closed in under the elytra gives the tip a a silver seam. Like most water insects, the Great diving beetle needs to come up for a new supply of fresh air. This air is taken in by bringing the tip of the abdomen to the water surface and then lowering it. However, some females do have smooth wing cases too!