Invertebrates
Diving Beetle
Dytiscidae · also called diving beetle, predaceous diving beetle, water tiger
A diving beetle is a pond’s top predator in miniature — a smooth, dark, air-carrying hunter that rows after anything it can catch, up to and including small fish. Its larva is worse: a long, jawed grub called the water tiger that spears its prey, pumps in digestive juices, and drinks the body out.
Overview
Diving beetles are the family Dytiscidae, several thousand species in still fresh water worldwide. Both the adult and the larva are fierce predators — insects, worms, snails, tadpoles, small fish. The adult swims with oar-like, hair-fringed hind legs, and because it is slightly buoyant it must actively swim down and hold onto plants to stay under.
Air and armour
An adult breathes air, carrying a bubble under its wing cases that doubles as a physical gill, drawing in dissolved oxygen as the beetle uses its store. It surfaces tail-first to refill. Many species also make defensive chemicals that make them foul-tasting or toxic to fish and frogs.
The water tiger
The larva — up to 6 cm in the biggest species — has huge hollow, sickle-shaped jaws. It stabs prey, injects enzymes that liquefy the insides, and sucks it dry, tackling animals far bigger than itself. Fully grown, it leaves the water to pupate in damp soil at the pond edge.
Status
Least Concern as a family — most species are common and quick to colonise new water. But several large species have declined in drained, intensively farmed landscapes and are now legally protected, hit by the loss of clean, fish-free ponds.
Diving Beetle: common questions
What is a water tiger?
A water tiger is the larva of a diving beetle. It is a long, narrow, six-legged aquatic grub, pale to brown, with a big head and a pair of large, curved, hollow jaws, and in the largest species it can reach five or six centimetres. It is one of the top invertebrate predators of a pond, seizing insects, worms, tadpoles, and small fish, and it feeds by driving its hollow jaws into the prey, injecting digestive fluid, and sucking out the liquefied insides. After weeks of hunting and growing, the water tiger leaves the water to pupate in soil and later emerges as the adult beetle.
Do diving beetles bite, and are they dangerous?
Large adult diving beetles can give a person a sharp, pinching bite with their jaws if picked up, and the water tiger larva can also nip, sometimes hard enough to break the skin and sting for a while. Neither is venomous or dangerous, and any bite just needs cleaning. The real danger they pose is to other pond life, since in a small pond or a tank of fish fry or tadpoles, diving beetles and especially their larvae can eat their way through a lot of young animals.
How do diving beetles breathe underwater?
They do not have gills as adults. An adult diving beetle carries a bubble of air, held under its wing cases and against the tip of its abdomen, and breathes from that store while submerged. The bubble also works as a physical gill, because as the beetle uses up the oxygen in it, more oxygen diffuses in from the surrounding water, extending the dive. Eventually the bubble shrinks and the beetle returns to the surface, breaking the film with the tip of its abdomen to take on a fresh supply.
Why has a diving beetle appeared in my swimming pool?
Adult diving beetles fly at night to find new water, and they locate it partly by spotting flat, shiny, reflective surfaces from the air. A swimming pool looks exactly like an inviting pond, so the beetle lands, discovers the water is chlorinated and has no food, and either dies or flies on. The same thing draws them to garden ponds, water troughs, wet greenhouse floors, and car roofs after rain. In a natural or wildlife pond they are a welcome sign of a healthy ecosystem; in a chlorinated pool they are just a stray.