Saturday, 26 June 2010

Dancing on water

Watch the surface of a piece of deep and still water and you may be rewarded by the sight of the dancing shapes of whirligig beetles. Like shiny black beads, their glistening forms catch the eye as they whirl and gyrate across the water’s surface. This characteristic dancing motion provides both their common and scientific names; ‘whirligig or whirlygig’ and ‘Gyrinus’ respectively. They are quite remarkable creatures, able in the adult form to exploit the surface tension of the still waterbodies they inhabit, and they show a number of adaptations to this narrow world.

The larvae of these water beetles live below the water’s surface. Predatory in nature (as is the adult form) they breathe through filamentous gills, a feature that frees them from the need to return to the surface to breathe. This may be why they are able to exploit deeper waterbodies than other water beetle species.

The adult form has two pairs of highly modified legs in addition to a more typical (for a beetle) front pair. The hind and middle legs are much reduced, thickened and have strong hairs that fringe their edge. Watch a flotilla of these beetles and you get the feeling that they zoom across the surface in a seemingly smooth and mechanical manner. Of course, it is the legs that are doing the hard work. These operate like switchblades, snapping open on the forward stroke but then folding in on the return stroke to minimise resistance against the water’s surface. In human terms the speeds attained by these beetles would approximate to 180 miles per hour – quite an achievement! Another key feature is the way in which each eye is split horizontally, with part above the water and part below. This allows the beetle to detect prey and potential predators in either of its two worlds at the same time.

If you see a group of these beetles then you will almost certainly being seeing the Common Whirligig which, as its name suggests, is the most commonly encountered species. In fact, it is thought to be more common than all of our other whirligig species put together (there are a dozen different species in Britain). While some of these have restricted distributions, the apparent scarcity of the others may reflect their habitat preferences, with most preferring to live amongst dense emergent vegetation. One species, the Hairy Whirligig, makes use of both running and still water and is nocturnal in its habits. Even if you find one of these rare species it can prove difficult to secure an identification; most can only be reliable identified based on the shape and structure of the male or female genitalia. Best just to admire them from a distance then! 

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