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Life in Freshwater

Great Diving Beetles (Dytiscus spp)

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Great Diving Beetle adultAdult Great Diving Beetle at the pond surface. Photographed from beneath the animal is reflected in the surface.

This is an impressive water beetle (and one of the largest) which grows up to 40 mm in length. The sleek, smooth and streamlined body is beautifully adapted for swimming. Like all members of the family the head fits neatly inside the thorax which again fits snugly against the abdomen to produce the refined outline. (compare this with the Screech Beetle) The thorax and abdomen are edged in a yellowish line and the dark upper parts have a greenish sheen. The female has a corrugated pair of wing cases whilstthe male is smooth. The legs are fringed with hairs to provide paddles for swimming. The male has a flattened area on the front leg which is lined with dense patches of hair. These act like suckers and hold the female during mating. The larva is long and narrow with a large head and needle-like fangs. The "tails" at the end of abdomen are paired and feathery. The larvae grow up to 60 mm. in length. This is one of many members of the Dytiscidae, a very large family of water beetles. They are commonly found in ponds that have dense vegetation. It may also be in canals and other static water. Where one is found others are usually about. They fly between ponds and so are easily dispersed. They are widespread across western and central Europe.

Larva of great diving beetle
Larva of great diving beetle

ECOLOGY: All Dytiscids are voracious carnivores. The larvae rear up the abdomen almost vertically and open their fangs ready to bite anything that comes near.

Head of the Diving Beetle larva
Head of the Diving Beetle larva, note needle-like fangs

The adult is a rapid swimmer and, like the larva, will tackle most aquatic life for food. Frogs, tadpoles and fish will be consumed as much as invertebrate animals. They will even be cannibals. After mating the females lay eggs near the bottom of the pond. Larvae grow and develop for up to two years before they burrow into the bank in the side of the pond. In the burrow they pupate. The beetles fly well between ponds providing good dispersal. They are also known to hibernate. They trap air beneath the wing cases when they come to the surface. The spiracles, openings into the breathing system, open upwards from the body into the bubble. Thus oxygen is removed and carbon dioxide put back in its place. Some of the carbon dioxide will diffused out into the water and oxygen diffuses in. This gill-like effect is called a plastron. However, due to the large size of the beetle there is usually insufficient gaseous exchange with the pond water and the beetle has to come to the surface to renew its air bubble.











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