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With approximately 3,000 species worldwide, Diving beetles are the largest water beetle family. The scientific name comes from the Greek dyticon, to dive, so Dytiscus means something like "diver". Dytiscus marginalis, the Great Diving beetle is the best-known representative. Diving beetles are thought to be descended from ground beetles and were therefore called Hydrocanthari.

Water predatory beetles, also called swimming beetles, are much more adapted to life in the water and swim much better than the "real" water beetles, the Hydrophilidae, which crawl more in the water than swim.

The most striking adaptation is the streamlined shape: head, thorax and abdomen (abdomen) flow smoothly into each other. They are often quite flat, making it easy for them to slip between aquatic plants and under stones. The legs are heavily modified: the front and middle legs for gripping and anchoring and the hind legs for swimming.

The hind legs are broadened and flattened like paddles, and usually provided with a single or double hairline, which can hinge in and out. To swim, the beetle moves its swimming legs forward as narrowly as possible during a forward stroke, usually extended lengthwise under the body. The hair hems are then collapsed, so that the friction resistance is as low as possible. During the backward, propulsive stroke, the legs extend wide like oars, with the hair edges folded out, so that there is maximum working surface. The legs move backwards simultaneously. Due to these adaptations, the beetles usually have difficulty walking outside the water, although some species can make a high jump. (Acilius, Laccophilus)).

Furthermore, while swimming, the front legs and often the middle legs are folded against the body to further reduce resistance. Some species have slots behind the thorax where these legs fit so that the underside is also streamlined.

When at rest, in most species the hind legs are held diagonally up along the body, like the feathers on a hunting hat.

The males often have widened parts in the forelimbs, with suction hairs that have small cusps, to better hold on to the female's pronotum during mating.

The head has long antennae and strong, sharp jaws with which the prey is killed and eaten.

Many species have (hind) wings with which they can fly, although not all species with wings are capable of flight.

Water beetles have no gills. Like land beetles, they have a system of branched air tubes (tracheae). Therefore, they must carry air with them to breathe. The Dytiscidae often store this air in the space between the abdomen and the elytrae, the subelytral space. The openings (stigmata) of the air tubes are located on top of the edge of the abdomen. The air supply simultaneously regulates the center of gravity and buoyancy in the water. Most of the time the beetles are lighter than water, so they have to swim or cling to plants or soil to stay submerged. When they release their hold, they slowly rise until the tip of their abdomen touches the surface. They then rest against the surface membrane with the triangle formed by the claws of the upwardly pointed hind legs and the hind tip. The elytra are lifted slightly, creating a space at the tip of the abdomen that is in contact with the outside air. Then the beetle starts breathing, the air in the tracheae and the subelytral space is refreshed. Sometimes the beetle lifts the elytra further and rises a little higher, causing a large part of the back to come into contact with the outside air. I have the impression that many Dytiscidae prefer to eat their prey in this position. Smaller species have little air under the elytra, but carry an air bubble on the abdomen. Sometimes the subelytral air is transferred into the bubble, which then becomes larger. Due to the physical lung principle, oxygen from the water enters the bubble, allowing the beetle to remain submerged for longer.

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THE LARVAE

The eggs of many Dytiscidae are inserted into plant parts. The larvae look completely different from the beetles, are pale yellow to brown, have an articulated body and a flat head, which has a pair of single eyes on both sides. They are always carnivorous animals with enormous voracity, which is caused by the need for rapid growth. Cannibalism is more the rule than the exception. Their jaws are sickle-shaped and have a closed groove running through them. They are inserted into the prey, the prey is injected with poisonous digestive juices and sucked dry. Some species can swim and hang breathing on the water surface with the tail end, where the two openings of the air tubes are located and where two tail appendages (cerci) are often present to support the tail end against the water surface. They molt a few times and then go ashore to dig a hole in which they pupate. Most beetles hibernate as adults.

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Literature: (see also: the Waterbeetle bibliography).

Schmidt, E. 2008 "Lebensgemeinschaft Dorfteich" (JUNGE 1885), ein Meilenstein der aquatischen Öko-Entomologie: das Beispiel "Gelbrandkäfer" (Dytiscus)The Community of the Village Pond (JUNGE 1885), a Milestone of Aquatic Eco-Entomology: the Great Diving Beetle Dytiscus as example Entomologie heute 20 (2008) 257-268
Retrieved 25 march 2024 from: https://www.zobodat.at/pdf/Entomologie-heute_20_0257-0268.pdf

Wesenberg Lund, die Biologie der Süsswassertiere

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