We’ve envied them, probably from the moment we realised that they could fly and we couldn’t; that they had wings and we didn’t. And, to rub it in, Mother Nature turned out two completely different, if equally ingenious, wing designs and flight techniques to enable insects and birds to take to the air in different ways. Here, we’ll take a look at what insect wings are made of and how they work.
Insect wings are made of two layers of chitin, which is pliable, resilient and tough, sandwiched together — the same substance that an insect’s exoskeleton is comprised of. They may be iridescently transparent and gleaming as cellophane — as in the wings of flies and dragonflies — or, glamorously coloured and patterned, as in butterflies. They look flimsy and seem so fragile as they twist and turn — but that’s just an illusion, for they can beat 250 times a second, twisting and turning around their axis with every single beat, creating and shedding furious little tornadoes (vortices) as they do. Structural strength is supplied by a fretwork of veins (the pattern is unique to each insect species) and, while most of the wing is “dead”, there are some nerve endings which are alive and sensitive. At rest, they don’t even look like typical wings — being flat, plane surfaces. But once they’re set into motion, they take on the shape of aerofoils, and provide the lift necessary to get what is essentially a most un-aerodynamic looking creature off the ground and zipping into the air.
Most insects have a single pair of wings, though many started off with having two pairs. Some ancient traditionalists like the dragonflies still do. In the beetle clan, the pair of forewings evolved into colourful wing covers called elytra, which are raised when the insect flies. They also contribute to a little lift. In others, like true flies, the rear wings evolved into stubby knobs known as haltres which provide stability in flight. Several four-winged insects such as butterflies, bees and wasps, hook or attach their rear and front wings together while flying so that they function as a single pair. Insect flight is exceedingly complex and has befuddled us for a very long time before we began figuring it out.
Perhaps, the simplest way to explain it is that insects fly in much the same way as helicopters do — blowing air below and away from them, thus providing lift and propulsion. While a helicopter tilts its nose downwards to achieve the correct combination of lift and propulsion (so that its vanes are angled correctly), the insect achieves this by twisting its wings around their axis like a figure of eight during every wing beat. A furious amount of energy is required to make these bulky, leggy little creatures take off and fly, and this is provided by the massive muscles in the insect’s thorax. Here, too, there are two different systems at work.
Four-winged insects like dragonflies have one pair of flight muscles per wing, which operates the wings directly through an ingenious hinging mechanism — one muscle is responsible for the upstroke and the other for the downstroke. The muscles are set vertically in the thorax, hinged to the base of the wings from the top. As each wing has its own set of muscles, it can beat independently of the others, enabling the insect to conduct astonishing flight manoeuvres. On the flipside, a dragonfly can’t beat its wings more than about 25 beats per second, though this doesn’t seem to affect its aerial performance at all — and it can fly perfectly comfortably even with just three wings. Insects like bees and wasps have two pairs of indirect flight muscles, which are connected to the thorax, horizontally and vertically, and their wings are attached to the sides of the thorax. Contraction of the vertical muscles tug the top of the thorax down and make its sides bulge out making the wings move upwards, and the contraction of the horizontal muscle arches the thoracic roof upwards causing the wings to move downwards. The system is so levered that a small movement of the thorax results in a relatively large wing movement and an amazing “click” mechanism (rather like the snapping “on” and “off” of an electric light switch) operating between the thorax and the wings speeds up the frequency of the wing-beats.
A unique bit of hyper-active muscle tissue called the fibrillar muscle, which automatically contracts after being stretched, ensures that the power supply to the wings continues well after the impulse from the central nervous system has stopped. The muscles can, thus, contract and relax much faster than the nervous system can command it to, enabling wing-beat frequencies of 250 beats per second. In addition, there are several other accessory muscles which allow the wings to twist and turn, enabling the insect manoeuvre so dazzlingly — and for a fly to land upside down on the ceiling! Like most engines, the flight motors of insects, too, need to be warmed up before they can function and some insects can disconnect their wings from their muscles (or simply shiver their wings) while vibrating these so that they achieve operating temperatures (between 30 and 40 degrees Celsius). This is why insects don’t usually fly on freezing winter days.
It really is astonishing to think of how a creature as fragile as a butterfly, with its flaky tissue-thin wings can take on transcontinental flights, somehow riding out storms and gales en route; it’s hard enough to imagine how, with that jerky yo-yo flight pattern, it manages to touch down with such precision on a wind-blown flower (like landing a helicopter on a bobbing cockle-shell boat!). Or, how that pesky housefly easily evades your flyswatter, zooms cheekily in front of your face and does quadruple back flips before landing upside down on the ceiling, yet again.
You can only give it a perfect 10.