September 22, 2021 4:50:56 pm
Recently, I waxed eloquent on the astonishing prowess of birds that hunted flying and moving prey — from glamorous peregrines and super-fast swifts to acrobatic flycatchers. Alas, they all have been completely outgunned — as have other terrestrial hunters (African wild dogs, lions and tigers) — by, worse, a creature that evolution designed perfectly 320 million years ago. They were much larger then — the dragonflies (and their slender cousins, the damselflies) — and we ought to be thankful for that!
Peregrines have a 23.1 per cent successful kill rate, hawks 22.5 per cent, owls 25 per cent, lions less than 30 per cent and wild dogs 67 per cent. Topping them all are (the ICSE topper-equivalent) dragonflies who score 95 per cent! Evolution got the design and engineering so perfect that no changes (except in size) were necessary.
Flight in most insects is powered by indirect flight muscles which are attached to the thorax, which are attached to the wings, making them vibrate and flutter. Dragonflies have very powerful direct flight muscles attached to the base of each wing that operate each wing. The muscle contraction causes each wing to flap — completely independent of one another if necessary — as the flight situation demands. This enables the dragonfly to fly forward and backward (by “standing up” perpendicular in mid-air and reversing thrust), upwards and downwards, hover and dart sideways and accelerate at tremendous rates.
“Counter-stroking” occurs when the forewings and hind wings beat 180 degrees out of phase with each other; which gives them one hell of a lift and enables them to hover and fly slowly. More thrust but less lift is generated by “phased stroking” when the hind wings beat 90 degrees ahead of the forewings — and is useful for high speed. With “synchronous stroking” the hind and forewings beat in rhythm enabling lightning acceleration and deft direction changes. Dragonflies clock 50 kmph, one report even claimed 90 kmph. Their body acts as shock absorbers. They may choose not to flap their wings at all and simply glide on air currents like many birds of prey.
The wings, made of flexible transparent chitin, often beautifully stained, are not flat, but lined with ridges — veins — adding to structural integrity and functional efficiency, and prevent the wing from warping and deforming. But the masterstroke is the little colour strip on the leading edge of the forewing of every dragonfly. Called the pterostigma, it is heavier than the rest of the wing and acts like a counterbalance preventing the leading edge from fluttering, going berserk and causing the insect to stall.
The “wrap-around” bulbous eyes of the dragonfly (giving it a field of vision of almost 360 degrees) comprise over 30,000 individual facets, each a miniature telescope that can see colour from orange to UV and are particularly sensitive to movement. The eye’s top section is especially sensitive to the blue range of the spectrum and the mid and lower parts to a wider spectrum. Patrolling low over the water, dragonflies are quick to spot an insect flying above, contrasted against the piercing blue of the sky.
Two hunting strategies are used: Tracking and interception. For the former, the insect keeps its target in its field of vision and spirals towards it, it’s advantage in speed eventually enabling it to nab it. In the latter, it must predict the spot where the victim would be and fly towards that spot — requiring more brainpower, but less energy and is the one the insects prefer. The target is kept in the eye’s sharpest-focusing section and the whole head can move around to enable this. We have to learn how to intercept a target — in dragonflies the ability is hardwired into their nervous system.
From a wingspan of 70 cm, dragonflies over the millennia have shrunk to a wingspan of just 16 cm. One theory suggests that this was because the oxygen in the air reduced from 35 per cent, 320 million years ago, to 20 per cent today, causing the insects to reduce in size. Another suggests that 150 million years ago birds and other flying reptiles evolved and out-competed large dragonflies in hunting larger prey, and so, only the dragonflies that hunted smaller prey survived. Whatever be it, we ought to be grateful, or else we would be ducking for cover.
Needless to add, biomimicry projects are desperately trying to emulate dragon flight. Many prototype “dragons” have been flown, but the real thing still blithely patrols the skies and waterbodies, grabbing insects in the barbed-wire basket that comprises its legs, chomping them up mid-air with their formidable mandibles. Then, it will land on a stalk, its head and great jewel eyes moving around as it looks for its next kill.
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