|Leach , 1815|
The swimming beetles (Dytiscidae) are a family of beetles that are described worldwide with around 3,200 species. They occur in Europe with 375 species and subspecies, of which about 152 species live in Central Europe .
The beetles are small (2 mm) to very stately (> 40 mm) animals of either inconspicuous black, brown, red-yellow or with a color-varying pattern of ribbons, spots and borders.
The Dytiscidae occur in different types of water; They can be found in almost every pond, lake, bog, river, stream, as well as in brackish water and in groundwater (here with receded eyes). Especially in places with uncontaminated waters.
They are beetles with an elliptical, flat arched body shape with a closed contour and a smooth body surface. Depending on how the species track down their prey, their bodies show considerable differences in terms of water resistance (search hunters, ambush hunters). The hind legs (swimming legs) of the different species are adapted to the movement in the water in different degrees of perfection. They are flattened and on the rails and the five foot phalanxes ( tarsi ) are covered with bristles that automatically spread out when the oar is turned. The insect can flatten it better than any human rower when it counter-blows, because the feet are connected to the rails so that they can rotate about the longitudinal axis. When the beetle pulls the "oars" backwards, the bristles stand out in a stiff row and offer the greatest possible resistance; if he strikes for the next stroke, the feet are turned and the bristles lie flat and do not hinder the propulsion. For this, the swimming legs are moved simultaneously and in the same direction (in contrast to the left-right alternating movements of the swimming legs of the water treaders ), while smaller species also use the middle pair of legs.
The animals are distributed all over the world, they can be found in almost all vegetation-rich waters. They colonize ponds, lakes, streams, rivers and sometimes even brackish water .
Way of life
The small swimming beetle species often do not need to swim to the water surface for weeks because the natural vegetation provides them with sufficient oxygen to breathe. The large species, on the other hand, often have to emerge in order to draw fresh air supplies. They take on a very characteristic position. They "hang" on the surface of the water in such a way that the rear end of the body protrudes slightly from the water and bend the abdomen a little downwards so that an opening is created between the wing covers and the abdomen through which the gas exchange takes place. The air penetrates between the folded skin wings, under which the respiratory openings ( spiracles ) are located on the abdominal segments , which open into the tracheal system . The exhaled air is also important for the swimming beetles, it helps them to compensate for the hydrostatic differences at the various depths. The amount of air taken in is adjusted so that the body has approximately the specific weight of water.
The larvae of the Dytiscidae attach themselves to the surface of the water with the help of special abdominal appendages and the last abdominal segment and take in atmospheric air into their large trachea via the rear pair of stigmas.
Many swimming beetles are good fliers and so they can change their whereabouts if there is a lack of food or a drying pool. The spread of the species and the colonization of new bodies of water occurs mainly by air.
The degree of filling (with water or feces) of a rectal blind sac (rectum ampoule) is also used to regulate buoyancy. This tube-like structure enables a weight change, which is very important for the transition from water to air and vice versa. If the rectal ampoule is empty, the beetle has considerable difficulties immersing and submerging after long flights. He only succeeds in this after swallowing water vigorously. Smaller dytiscids have developed a complex, instinctive movement pattern in order to overcome the surface tension of the water, which is a significant obstacle for them.
As carnivores, beetles and larvae prey on tadpoles, fry, insect larvae, other small aquatic animals and also carrion. While the beetle is eating its prey, the larvae have inherited extraintestinal digestion from their ancestors, the ground beetles. You can neither chew nor swallow. Their sharp, pincer-like mandibles are traversed by a fine canal that opens outwards near the tip of the mandible. If the larva hits its upper jaw into a victim, a trypsin-containing digestive secretion is immediately injected through the canal , which leads to rapid paralysis and the initiation of pre-digestion of the prey. The liquefied body content of the prey is then absorbed.
The swimming beetles have paired, often very complex glands in the front chest and abdomen, which produce typical defense materials and secretions in different combinations. These have either anti-feeding, anesthetic, toxic (defense), antimicrobial (against bacterial and algae infestation of the beetle's body), fungicidal (against fungal infestation) or wetting. The wetting effect of their secretions makes it easier for small swimming beetles to submerge.
The sexes differ quite clearly from each other in many species ( sex dimorphism ). The males have strongly widened forefoot limbs with suction cups of different sizes on the underside, while the wing covers of the females are often strongly ribbed.
After copulation has taken place under water, the eggs are either glued to the plant substrate, depending on the species, pushed superficially into plant tissue with a laying apparatus or sunk deeper into a plant with a powerful ovipositor.
The larvae always live in the water and go through three stages of development. To pupate , the larvae of most species leave the water and dig burrows in the ground. When moving on land, they bite their mandibles into the ground and drag their bodies. Depending on the temperature, the beetles hatch after two to five weeks and then hibernate.
There are some quite long-lived species among them that even survived several years in captivity.
European subfamilies and genera of swimming beetles with selected species:
- Common furrow swimmer ( Acilius sulcatus )
- Narrow-band broad-winged diving beetle ( Graphoderus bilineatus )
- Yellow-brown dwarf swimmer ( Hydroglyphus pusillus )
- Six-spotted dwarf swimmer ( Hydroporus palustris )
- Smooth ball float ( Hyphydrus ovatus )
- Dytiscidae. Fauna Europaea, accessed April 16, 2007 .
- B. Klausnitzer: Beetles in and on the water, ISBN 3-89432-478-3
- Jiři Zahradnik, Irmgard Jung, Dieter Jung et al .: Beetles of Central and Northwestern Europe. Parey, Berlin 1985, ISBN 3-490-27118-1