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The 'bumblebees can't fly' lie

Billions of insects take to the sky every day, but we've only recently discovered how their flimsy wings support them. Dr Karl lifts the lid on insect flight.

Many urban myths and half-truths roam our human consciousness. One oft-repeated fairytale runs that according to the laws of science, the bumblebee shouldn't be able to fly at all — indeed, not even be able to get off the ground. The reality is, as you probably guessed, that insects can fly — some as high as Mount Everest.

First, how did this story about the bumblebee pop into existence?

The typical sentence runs: "Didn't an aerodynamicist prove that bumblebees can't fly?"

According to John McMasters, who back in the 'good old days' was principal engineer on the aerodynamics staff at Boeing Commercial Aeroplanes, it seems the aerodynamicist of the myth was probably an unnamed Swiss professor famous in the 1930s and 1940s for his work in supersonic gas dynamics. The aerodynamicist was having dinner with a biologist. In the idle chit-chat, the biologist noted that bees and wasps had very flimsy wings — but heavy bodies. So how could they possibly fly?

With absolutely no hard data, but a willingness to help that overcame good dinner party etiquette, the aerodynamicist made two assumptions in his back-of-envelope calculations.

The first assumption was that the bees' wings were flat plates that were mostly smooth (like aeroplane wings). The second assumption was that as air flows over an insect's wings, it would separate easily from the wing. Both of these assumptions turned out to be totally incorrect — and the origin of our myth.

The aerodynamicist's initial rough calculations 'proved' that insects could not fly. But that was not the end of the story.

Of course, being a good scientist, his sense of curiosity got him interested in this problem. Clearly, insects can fly. He then examined insect wings under a microscope and found that they had a ragged and rough surface. In other words, one of his assumptions was way off.

But by then, overzealous journalists had spread the myth he had inadvertently created. The story had flown free, even though the bumblebee supposedly couldn't.

In fact, once our scientists began to look closely at how insects fly, they discovered astonishing details. For example, the aerodynamics used by flies gives them the amazing ability to do a right-angled turn in less than 50 milliseconds.

This problem of how insects fly was so hard to solve, that the mystery was solved only in the last 20 years.

As insects flap and rotate their wings, vortices (or spirals) are created on the leading edge of their wings. These vortices stay 'stuck' to the insect wing (unlike the vortices created by aeroplane wings, which slide off). These vortices then produce lift which keeps the insect airborne.

To keep themselves aloft, fruit flies sweep their wings through a very large angle (145 to 165 degrees).

But getting back to bees, some species of bees have rather shallow strokes (less than 130 degrees). But to compensate, the bees have a very high beat frequency (230 per second).

Bees have a special trick to increase their power output enormously. They greatly increase their stroke amplitude to 190 degrees, but increase their beat frequency only slightly to 235 per second. As a result, bees can fly at altitudes higher than nine kilometres with a normal load, or alternatively, carry enormous loads greater than their own body weight.

So how high can insects fly?

Butterflies can flap joyfully at one kilometre above the ground, while flies can reach 1.5 kilometres. Midges, aphids and wasps can go a little higher, while ladybug beetles can reach 1.8 kilometres. Gypsy moths (mostly males looking for female partners) have been found three kilometres above the ground. Spiders drift up to 4.2 kilometres above the ground, not using any power at all. They simply float on long thin silk strands carried by the wind. Termites (yep, the guys that eat the wood in your walls) have reached six kilometres above the ground. And bees can do nine kilometres.

You can't see them, but in the summer there are huge numbers of insects flying on many different pathways at different altitudes looking for food or a mate. Unfortunately, nature is bloody in tooth and claw — and most of them die.

How many of them are up there? Well, start by mapping out on the ground an invisible 'box' that's one kilometre by one kilometre, and that reaches all the way up to the heavens. It is estimated each month in summer, some three billion insects would pass through this invisible box.

As least we know that all the insects seem to be as busy as the bees.

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