The Whirligig Beetle
The Lower Knapp pond has a good variety of life beneath the surface, but one
the jewels can be seen on the surface. Formula one dodgem cars can be spotted on
a sunny Spring day, like diamonds moving erratically on the water surface.
They are the amazing “Whirligig Beetle”
They measure 5-8 millimeters, and their colour is usually steel grey or bronze, but the air bubble they carry around sparkles in the sunlight. They produce a water repellent outer waxy layer, making them difficult to hold with fingers, like a fresh orange pip. This causes problems for the male “Whirligig” during mating, and he has developed pairs of suckers to hold on to his partner!!
They feed mainly on small insects that have landed accidentally on the water surface. The beetle’s short plump antennae can detect the slightest movement on the water surface, using a form of echolocation. A pair of long fore limbs grasp the prey.
The fascination of the “Whirligig” is it’s incredible speed. The body is flattened on the water surface, with middle and hind limbs flattened also. These limbs have numerous long hairs (setae) acting as super efficient paddles. Speeds of some “Whirligigs” (there are 700 species worldwide) reach 144 centimetres per second. Apparently, these beetles have an 84% thrust efficiency, the best known in the animal kingdom. (not a lot of people know that!!). The thrust potential of the hind leg differs by as much as a factor of 40 between the forward and backward stroke. This is similar to Olympic oarsmen, but it is the speed of the strokes that is astonishing……..50-60 simultaneous strokes per second!! Aided by the waxy outer layer reducing surface tension, their acceleration on the water surface is impressive. They are capable of spinning at speed on the surface, as a survival strategy, hence the name “Whirligig”.
The eyes of the beetle are divided, one half looking along the surface and above, and the other half looking downwards, for predators below the surface. If these beetles are disturbed, they can fly off the surface, or dive below the surface taking an air bubble (plastron) with them. This air bubble is held by fine hairs, and enables the “Whirligig”, and other beetle species to survive for long periods beneath the surface, as oxygen from the water diffuses into the bubble, replenishing the “air tanks”. “Whirligigs” will also emit a foul smelling fluid, like rotten apples, if disturbed, to put off predators.
As I read about creatures like the “Whirligig Beetle”, I wonder at the humans who researched “thrust efficiency”, or smelt the “rotten apple discharge”, and other fascinating facts.
First published in SVA magazine 2019