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Why are flies so hard to swat?


And what does that have to do with Foucault's pendulum? Martin Harris has the answers in the latest in our 'Why is it So?' series.


The bluebottle fly shows off its aerobatic skills. – Heidi & Hans-Juergen Koch/Minden Pictures/Corbis

Oh to have been a fly on the wall that historic day in 1848 in the workshop of French physicist Leon Foucault.

Foucault was working at his lathe. Clamping one end of a long, thin metal rod into the tool, he happened to give the other end a good twang, setting it vibrating vigorously up and down like a just-used diving board. But as Foucault slowly rotated the lathe and the rod, he noticed something striking. The up and down vibration of the rod didn’t change. Even when the lathe had gone a quarter turn, so one might expect to see the rod vibrating horizontally, it didn’t veer from its diving board imitation. No matter how far he rotated it, the vibrating rod simply kept on bouncing straight up and down, somehow remembering its original direction of motion.

As Foucault crossed his workshop to try a follow-up experiment, the fly on the wall might have chuckled. It had long ago put to use the phenomenon Foucault had just discovered. But back to Foucault. From the chuck of his bench drill, he now suspended a spherical weight on a length of piano wire, then he set it swinging like a pendulum. He started the drill making the wire rotate, but as with the rod, the pendulum’s motion didn’t - it kept swinging in the line of its original plane.

Three years later, the French physicist demonstrated the same effect on a grand scale. In the spring of 1851 he suspended a 28 kg brass-coated lead weight on a 67 metre-long wire from the dome of the Panthéon in Paris, and then carefully set it swinging. Within an hour, just as Foucault had calculated, something strange was clearly happening. The plane of the pendulum’s swing seemed to rotate slowly clockwise.

In fact, the plane of the pendulum swing had not rotated; the Earth had rotated beneath it. Foucault had made the first demonstration of the Earth’s rotation that didn’t rely on observing the motion of stars.

He had also demonstrated to the world a fundamental property of vibrating objects. As if programmed with some sort of memory, they maintain their original direction of motion even when the material that is vibrating changes orientation.

Dragonflies literally eat flies for dinner thereby claiming the crown
for 'most accomplished aerobat'.

Following Foucault’s pendulum, human engineers began to take advantage of this memory of motion. It wasn’t just exhibited by vibrating objects, but also rotating ones. Because of inertia, spinning objects try to maintain their axis of rotation even if they are pushed.

But the darned fly, that Foucault no doubt tried very hard to swat, first put these principles to use more than 250 million years ago. Tucked under its wings was things that act in a similar way to what Foucault observed on his lathe: halteres. These are tiny vibrating club-shaped structures that help the fly keep itself on an even keel despite being buffeted by air currents or through abrupt direction changes. Halteres evolved from a second set of wings present in the flies’ ancestor. (see Winging it with the history of flight).

An electron micrograph of a bluebottle fly showing its wing structure. – Science Photo Library/Getty Images

How do the halteres help the fly to navigate? Just as Foucault’s oscillating pendulum resisted the twist of the rotating planet, the halteres try to oscillate in the same plane when the fly changes direction. Nerves at the base of the haltere stalk sense the tug on these tiny pendulums as the fly twists and turns. This provides “feedback”, an all-important phenomenon, whether it be in the human vestibular system in the ear that keeps us upright, in the angular momentum of a gyroscope on an aeroplane, or on a fly racing around the room, adroitly dodging all attempts to swat it.

Not all aerobatic insects have halteres. The dragonfly relies on its huge eyes, which take up almost 80% of the volume of its head, to keep track of its position in space as it hovers over pools. Which approach is more successful? Flies greatly exceed dragonflies in number. But on the wing, dragonflies literally eat flies for dinner thereby claiming the crown for “most accomplished aerobat”.

The fly's club-shaped haltere, a miniature vibrating Foucault pendulum. – STEVE GSCHMEISSNER/Getty Images

Flies were the first to miniaturise these vibrating Foucault pendulums – or reciprocating gyroscopes, as the Institute of Electrical and Electronics Engineers now calls them – but today, tiny silicon versions can be found in many electronic devices that need to sense rotation: image stabilisation systems in digital cameras that help keep photos sharp, toy (and full-size) helicopters, robots and smart phones.

Chances are you have a fly gyroscope vibrating rapidly in your pocket this very minute.

Martin Harris is a former science teacher and inventor who holds patents on optical devices used in cancer detection.
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