NATURE
Hoverflies of spring
RANJIT LAL
|
If you want to know about precision flying, watch the hoverflies in action.
|
Their USP is flight: Hoverfly in action. Photo: Ranjit Lal
IT is in spring that you really notice them, especially in the dazzling shafts of sunlight slanting through the emerald green and dappled leopard shadow patterns of Delhi's Ridge: motes of gold and gossamer, suspended motionless, that suddenly whiz backwards or forwards or up or down or sideways and then return to their original station: hoverflies or flower flies as they are also called. At first just a few and then larger numbers, some bold enough to materialise right in front of you and eyeball you, mother-of-pearl wings shimmering. Now you can admire their striped black and gold bodies and colossal coral orange eyes — even if you do end up squinting ferociously in order to do so.
Last season I spent hours trying to photograph them — easier said than done and a tough test of patience and perseverance. Back home after maddening photo-sessions I decided to find out more about them and their kind and naturally dig up what dirt I could on them, because that's what journalists do.
Life history
They belong to the immense fly clan. The family has between 5,000 and 6,000 species that we know of. In temperate lands they are summer-loving insects, and emerge in swarms. Here in Delhi, I've seen them hovering over the flower beds in winter, but spring is when they really catch the eye. As larvae as in adult form, depending on the species, their lifestyles can be hugely diverse. Like the larvae of drone flies (belonging to the same clan), some are brought up in muddy and polluted water and are equipped with long extendible snorkel-like tubes to breathe while immersed in liquid muck. The flies may even lay their eggs on soiled human bodies — for example the vomit around a child's mouth. If the eggs are swallowed, the larvae may grow in the intestinal tract and actually feast on human tissue.
Some larvae hatch and live in the nests of wasps or ants, feeding on the excreta and thus providing a sanitary service. Others eat bulbous plants, a practice that we of course do not approve of. Thankfully there are others with better and more useful culinary habits: a large group of hoverfly larvae are vigorously carnivorous and thrive on aphids, thrips and other plant-eating insects. Each hoverfly maggot may account for as many 60 aphids in a day, sucking out the soft, easily digestible body contents of their unfortunate victims. Thus they are natural pest controllers.
Important pollinators
The adults too are a diverse lot. Some are furry as bees, others sleek as wasps and some of the latter may even arc their backs and try and stab you with their posteriors like the fiercest wasps, but happily they are harmless. But birds taken in by the subterfuge will give them a wide berth. Nearly all of them are dependent on flowers — for the nectar and pollen that give them the energy required for their hectic highflying lifestyle. They are hugely important as pollinators, ranking only second to bees and wasps in this vital function. The males patrol air corridors they have claimed as their own and will brook no incursion into their airspace, unless by a female showing interest. I've watched them dart at each other fearlessly to drive interlopers away, and then resume patrol and sentry duties with dogged determination.
Puzzle of flight
For me, their biggest USP is flight. It has also been a subject of fascination for scientists wracking their brains trying to crack the puzzle of insect flight in general and the hoverflies' dazzling acrobatics in particular. Conventional aerodynamics (which explains how birds and airplanes fly) does not apply here. Insects use unsteady aerodynamics to keep aloft. At last, after creating a mechanical model of a jumbo hover fly and immersing it in two metric tons of mineral oil (to keep the physics the same — for a real hoverfly, the air is as thick and syrupy as mineral oil is for the model) and flapping its wings just as a real life hoverfly does, the secrets of hoverflight were revealed.
Lift was generated in three different ways. First it was found that vortices of air were created by the downstroke of the wings, moving along the upper wing to the tip, like little tornadoes turned sideways on. These shoved the air downwards behind the trailing edge of the wings and so created lift. The vortices were created by the steep angle at which the wing was briefly placed, which if prolonged would lead to a stall. But the lift created by this method, called the delayed stall, was found to be insufficient to enable the hoverfly manoeuvre the way it does.
Then it was found that the hoverfly also twists its wings as they move from upstroke to downstroke, thus creating lift in the same way as a viciously backspun cricket or tennis ball does. In addition this rotation of the wing also caused an air vortex to come spiraling off the edge of the wing — rather like a slow-moving rowboat leaves swirling eddies of water in its wake.
Wizardly control
Ingeniously some of the energy contained in this vortex was `recaptured' before it could swirl beyond reach, by the subsequent wingstroke and was the third contributor to lift. It is these latter two, the rotational lift and `wake capture', which gave the hoverfly the ability to change course in a wink of an eye and give it its wizardly control over flight. Miniscule changes in the timing of wing rotation could even produce `negative lift' enabling the insect to drop height startlingly.
No other insect I can think of flies with such, almost geometric precision as does the hoverfly. Butterflies skedaddle skittishly all over the place, bees, wasps, mosquitoes and even ordinary houseflies sway and sashay erratically before touching down and even dragonflies are not as neat and pert in their movements. For precision flying, it's got to be hoverflies.
Printer friendly
page
Send this article to Friends by
E-Mail
Magazine
