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July 2013 whiligig beetles

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Whirligig Beetles

Dick Warner explains the apparently madcap behaviour of a common pond beetle

Whirligig beetles are among the strangest forms of wildlife likely to turn up in your garden pond. Small, dark beetles usually seen in little clusters swimming in mad circles on the pond surface. If you disturb them they disappear under water. If you manage to catch one it will give off a very strong smell, which is its defence mechanism.

Why do they swim in circles? How do they breathe under water? How did they get to your pond in the first place? Whirligig beetles are predators. They feed on things like midge larvae and small invertebrates. In order to focus on both surface and sub-surface prey they have two pairs of eyes, one to see under water and one to see in the air. The whirling is apparently a way of detecting prey. They do it to create small ripples and when a midge, for example, lands on the water surface they can sense the change in the pattern of the ripples and home in on it.

Adult whirligig beetles, like all water beetles, have no gills and breathe air. In order to survive under water they capture a bubble of air in special organs on the tip of the abdomen and carry it around with them. They swim using legs that are modified to act like oars and can stay under water for a remarkably long time. Eventually, however, the air bubble runs out and they have to make a quick trip to the surface to replenish it. Leaving the cover of the water plants they hunt in to do this is a dangerous moment for them because many species of fish like to eat water beetles.

Their life-cycle is equally unusual. The larvae are also under-water predators but they look totally unlike the adults. They are long and thin with filaments that stick out at the side and make them look a little like under-water centipedes. The filaments are gills so the larvae can breathe under water and don't have to risk a trip to the surface to grab a bubble of air. But they do crawl out on to the land and spin a cocoon for themselves when it comes time to pupate.

The adult beetles that hatch from the pupae can fly as well as swim. But they will normally only take to the air once a year, at night in late summer or early autumn. Then they fly, often for considerable distances, in search of new ponds, ditches or canals to colonise. Other water beetles, such as water boatmen, which are technically bugs related to aphids, not beetles, and the diving beetles do the same thing and this is how they can suddenly appear and, equally suddenly, disappear from your garden pond.

There are actually many different species of whirligig beetle - over a hundred and eighty worldwide of which at least ten occur in Ireland. But the Irish species are all very similar and telling them apart is the business of skilled entomologists rather than pond-keepers. If they turn up in your pond, enjoy their antics, reinforced by the knowledge that they're doing something to reduce the number of midges, but accept the fact that they may not be visiting for very long.



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