SUBJECT MATTERS OF EXPERIMENTS AND OBSERVATIONS OF WATER ANIMALS IN SCHOOL AQUARIUM VI (REARING OF PREDACEOUS DIVING BEETLES)
Abstract
Aquatic beetles in the beetle family Dytiscidae are known as predaceous diving beetles. They all have a smooth, oval shape with strong, paddle-like hind legs that propel them easily in water.
A large bubble of air under the beetle wing covers (elytra) allows them to suck in a bubble of air from the water surface, as well as breathe underwater for long periods of time without coming to the surface. A suitable species for breeding in aquarium is Great Diving Beetle (Dytiscus marginalis).
It is a voracious predator, this beetle hunts a wide variety of prey including small fish. Interestingly, it is very easy to distinguish between the sexes in Dytiscus marginalis as females have parallel grooves running down their elytra (back or top surface), whereas males have smooth elytra.
But this character is not 100% reliable, because females sometimes have also smooth elytra. Reliable distinction is made by the conspicuous sucking discs at the end of the first pair of legs.
The aquarium equipment should be as follows: water to a depth of 20 cm or greater, ideally closer to room temperature. Underwater surfaces to rest on, such as plants, rocks, or twigs.
A cover on the top is very important (the beetles can fly). A piece of floating wood will do, or a rock that reaches above the water.
No water filtration or aeration equipment is necessary. Both adults and larvae are voracious predators feeding on anything in their reach including larval fish.
The healthiest and most entertaining foods for diving beetles are fresh invertebrates, we can use also meal worms, earth worms or small pieces of lean meat. Diving beetles can attack fish (sick and injured especially) even if they are larger, and eat them.
Diving beetles also have a fascinating defence mechanism: they secrete toxic substances that can make vertebrates, such as fish, sick if both are kept in a shared environment. Eggs are laid in or on vegetation beneath the water surface, or on other objects below or just above the surface.
The larvae develop over a period of a few to several weeks, then leave the aquatic habitat to pupate in cells that they construct in the soil of protected areas nearby (an aquaterarium must be provided to successfully pupation of larvae). The pupal stage lasts 5-14 days, after which the adult emerges and usually re-enters the aquatic habitat.
Some experiments on, and observations of, larvae and adult predaceous diving beetles in an aquarium, inspired by cited literature sources, are presented in this contribution.