Swimming in Air
Anyone who has ever seen some video footage on how the marine snail known as the “Spanish Dancer” (Hexabranchus sanguineus) or such dorso-ventrally flattened fishes like skates and rays move under water (the manta ray, often even leaping into the air) or how penguins dart through the water in pursuit of little fish will agree: they fly under water.
And that’s no easy task, because the density, i.e., the viscosity of water is much greater than that of the air. You can run more easily in air than in water and a ball travels much further in air than it does under water. The problem is greatest for the smallest of organisms, but even they have some representatives that “fly under water” like, the tiny pteropod snail known as sea butterfly (Limacina helicina) or a group of tiny parasitic wasps named “fairyflies”. These wasps lay their eggs into those of aquatic insects, e.g., water beetles. The most famous underwater flyer is the tiny mymarid wasp Caraphractus cinctus: it uses its thin and fringed wings as oars, but really flies under water in search of eggs that it can lay its own eggs into and it can stay under water for up to 2 weeks.
For aerodynamic reasons and to meet the different requirements of different insects, their wings come in all sorts of shapes and sizes, ranging from long and slender blades as in dragonflies to the sheet-like, colourful appendages in butterflies. Mechanisms of folding and tucking the wings away under special covers, when present, are equally diverse: from the simple roof-like position of that of lacewings to a pattern found in earwigs, which is so complicated that one is reminded of some origami (or a collapsible mini-umbrella for a lady’s handbag). But when we look at the wings of the dwarfs amongst the insects, the flying “Lilliputians”, we find one common design serving species as different as thrips, beetles and wasps. This remarkable convergence in wing types suggests that identical environmental pressures in combination with the insects’ small body size must have been at work and led to a common type of flight – no doubt brought about by the problem of the air’s viscosity.
While the problems for the larger, more powerful insects are principally to stay aloft and to exert maximum control over lift, speed and direction, those of the airborne midgets are quite different. With body sizes of less than 1´mm, they are not in danger of “falling down”. They find themselves in the midst of a thick soup and getting through it is their concern. This microscopic insect aeroplankton rarely sports wing that are longer than a few tenths of one mm. Each such tiny wing has a central rib with a short comb-like row of hairs facing anteriorly and a larger number of stiff, overlapping bristles projecting posteriorly. Wings of this type are like oars. The insects use them to push against the viscous medium air and “swim” in the air, known as a “clap and fling” The fringe of hairs on the wings reduces the drag, which would have been considerable had the wings been broad and solid. Still, I suppose they themselves must be thinking they are flying in air, but we know better now, don’t we? And for those who want to take a look at such of tiny winged insects
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2018.
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