The Natural Enquirer: The ‘magic’ water film
In a shallow, quiet inlet of our lake, with lily pads, old logs and an occasional great blue heron, scarcely causing a ripple in the small bay, a gathering of creatures runs about on the surface without falling in. Little Saint Francises walking on the surface, little magicians, somehow staying on the top when by their weight they should have sunk, they, the whirligig beetles and water striders are a living demonstration of the wonder of the surface film.
It is an invisible film which has no substance other than that of the water itself. It is always there, yet is quite unseen, except when its presence is demonstrated by some creature casually walking across it, when properly, animals which are heavier than water should sink. They skitter in a light footed manner, and only submerge when they wish to do so, when they do, they carry with them a supply of life-giving air down stairs.
Here in a small cove of a Northwoods lake, as well as out on the open water, dwells a miniature world of necromancy, efficiency and miracles. Water, like everything else, is composed of molecules which are in rapid motion. Every molecule with its kinetic energy attracts every other molecule, with the result that a substance even in the state of water stays together except when heated, at which time the molecules in vapor or steam are cast off into the air.
Water does not normally fly apart in disintegration. This is true within the body of water, but something else happens at that point at which the liquid ends in a level surface and the air begins. Here are molecules exerting a pull on every other molecule from all sides, except those which form the top surface. The pull is down, therefore, but not up, because the water molecules all end at that point. The downward pull of gravity and the downward push of air pressure create surface tension. It accounts for the way an insect’s lightly set feet actually press a dent or dimple into it. It permits the racing and chasing of the whirligig beetles and the floating of leaves and dead insects upon the surface until gravity eventually pulls them under. The film is a magic carpet spread invisibly upon the top of the water, and upon every pool and puddle, all over the world.
The realm of the whirligig beetle is on that fragile plain of the surface film. The beetle seldom is a solitary creature, but travel in packs. They are all as furiously hungry as wolves and as carnivorous as Lupus himself, only on a smaller scale. Shiny blue-black, half an inch long, nicely boat-shaped with an elegant streamlining which gives the least resistance against the drag of the water, the whirligigs dominate the small bay. They are small masters of the Northwards lakes. Swimming in circles with such speed that they become a glittering blur, they snap to a halt, as if at a traffic signal, all of them at once, as well regulated as flocking blackbirds. Lying flat on the water, they row with large front legs shaped like oars, while two other pairs of legs are folded snugly against the underside of the beetle’s shell.
Since the whirligigs are avid hunters, they are equipped not only with two good eyes, but with the equivalent of four, each eye is divided so that the whirligig can see above the water and below the surface at the same time, thus presenting no difficulty in finding food in one realm or the other. On the quiet water, the fleet of black beetles seems lifeless, simply floating like drifting seeds upon the surface film and occasionally striking a metallic spark from the sunshine, as one glossy, submarine-shaped back twirls aimlessly like a motor boat becalmed. But, as a hapless fly suddenly loses its balance from a twig above the water and tumbles in, wetting its wings beyond immediate recovery , the whirligigs come to life in a carnivorous frenzy.
As the fly kicks about on the surface film, the pack of beetles comes at it like a fleet of racing boats. A swirling mass of glittering black, they dart at the fly, tear off mouthfuls, perhaps dive under the water to escape the others which covet their bite, return like surfacing submarines, charge in again for more. As suddenly as it began, the bloodthirsty frenzy is over. The beetles pause placidly, twirl aimlessly about. Only the floating wings of the fly remain. Now and again a whirligig, obviously of short memory, rushes over to nip tentatively at a floating wing, making certain that all the meat is gone and there is no life left.
The whirligigs are creatures of the surface film, but they can live below for a length of time by simply carrying an air bubble down with them. It is a sort of portable aqua-lung which can be drawn upon for some time before replenishing with another bubble or by simply popping up to the top and staying there for a while. The whirligigs, besides, have on the underside of the body a quantity of tiny short hairs which hold a film of air. This helps to keep the beetle buoyed on the surface film and facilitates speed; none of this, however, is used for underwater breathing.
Peter Dring is a retired nature biologist and phenologist who lives in the Land ‘O Lakes area. To comment on this story, visit the “Outdoors” section of StarJour nalNOW.com.