Invertebrates
Great Diving Beetle
Dytiscus marginalis · also called water tiger
Drop a net into a weedy pond in Europe and you might pull up a smooth, dark, oval beetle the size of a thumbnail that immediately tries to bite. This is the great diving beetle, and in the small world of the pond it is a top predator, hunting animals many times its size.
Overview
Dytiscus marginalis lives across Europe and northern Asia in clean still fresh water with plenty of plants and few fish. The adult is a streamlined 3 cm beetle, dark olive with a yellow border, and an aggressive predator of tadpoles, newt larvae, and small fish.
Carrying its air
It cannot breathe water. It surfaces tail-first, traps a bubble of air under its wing cases, and carries that supply down, renewing it every few minutes. Under winter ice it stays active, breathing from trapped bubbles.
The water tiger
The larva is a pale, fast hunter longer than the adult, with hollow sickle jaws that inject digestive juices and then suck the prey dry.
Flight and mating
On warm nights adults fly between ponds, sometimes crash-landing on wet roads and greenhouse roofs. The male grips the female during mating with suction cups on his front feet.
Status
Least Concern and widespread, though local populations fall where ponds are lost, polluted, or overstocked with fish.
Great Diving Beetle: common questions
Can a great diving beetle bite a person?
An adult can give a noticeable pinch with its jaws if it is picked up and handled roughly, but it is not dangerous and rarely breaks the skin. The larva, the water tiger, is a different matter, as its long curved jaws can deliver a sharp, stinging bite along with a dose of digestive saliva, and people who net pond life sometimes get nipped. Neither is a medical concern. The beetle's chemistry is more of an issue for would-be predators than for humans, since its defensive secretions taste foul.
How does the beetle breathe underwater?
It does not extract oxygen from the water at all. It carries its own air. Roughly every few minutes the beetle rises and breaks the surface with the tip of its abdomen, opening a gap under the rear of its wing cases and refreshing a bubble of air held against the breathing pores, or spiracles, on top of the abdomen. That trapped bubble also works a little like a gill, drawing in some dissolved oxygen from the water as the beetle uses up what is inside, which extends each dive. In winter, under ice, the beetle stays active and takes air from bubbles trapped beneath the surface.
What is a water tiger?
It is the larva of a diving beetle, and the name fits. It is a slender, mobile, translucent creature, in the great diving beetle up to about 6 centimetres long, with a small head carrying two enormous curved jaws. Those jaws are hollow. When the larva seizes a tadpole or an insect, it pumps digestive enzymes down the grooves in its jaws into the prey, waits while the tissues dissolve, and then sucks the resulting soup back up the same jaws, discarding an empty skin. A single pond can hold many water tigers, and they are significant predators of amphibian eggs and larvae.
Why do diving beetles turn up far from water?
Because they disperse by flight. When a pond becomes crowded, starts to dry, or freezes solid, or simply on warm still nights during the breeding season, adult great diving beetles climb out, dry off, and fly, sometimes for kilometres, to colonise new water. They find water partly by looking for horizontal surfaces that reflect the sky, which works well for ponds but also fools them into landing on wet tarmac, the glass of a greenhouse, a swimming pool cover, or the shiny roof of a car. A beetle found flailing on a driveway after rain has usually just made that mistake.