Why flies are so hard to swat

Michael Dickinson is a genius.

At least, in 2001 the University of California, Berkeley, professor received one of the $500,000 “genius” grants given annually by The John D. and Catherine T. MacArthur Foundation to creative individuals “who provide the imagination and fresh ideas that can improve people’s lives and bring about movement on important issues.”

That’s one way you know he’s a genius. The other is that he has just answered a question that has bedeviled human beings since the dawn of time: “Why are flies so hard to swat?”

Dickinson has built his entire career around the study of the flying abilities of insects in general and flies in particular.

“Flies are the most accomplished fliers on the planet in terms of aerodynamics,” he says. “They can do things no other animal can, like land on a ceiling or inclined surface. And they are especially deft at takeoffs and landings–their skill far exceed that of any other insect or bird.”

How do you study insect flight? Multiple ways.

For instance, Dickinson sometimes puts flies in a virtual reality “flight arena.” He ties them down and then records their wing motions as they react to visual or mechanical stimuli, or even various food odors.

He has also constructed ten-inch-long models of fruit fly wings and immersed them in mineral oil so he could study the currents and vortices set up by their wing motion.

As good experimentation is wont to do, his work occasionally overturns long-standing misconceptions.

For example: to turn, a flying creature has to generate enough torque to offset the inertia of its own body and the friction of the air. Scientists had long assumed that the latter was most important for small insects, for whom the air, they figured, must feel like syrup, whereas inertia was more important in larger creatures like birds.

Dickinson discovered otherwise: inertia turns out to be the most important force flies have to overturn, as well, and this contributes to their amazing turning ability.

Fruit flies make subtle changes in the tilt of their wings relative to the ground and the size of each wing flap to turn, then have to create an opposite twisting force to stop the turn so they don’t spin out of control. If the viscosity of the air were the main force they had to overcome, there’d be no need for that opposite twisting force: the air itself would stop the turn.

More recently, Dickinson combined robotic modeling with slow-motion video to at last answer the question of how honeybees, heavy insects with short wing beats, generate enough lift to fly, in apparent defiance of the calculations of aeronautical engineers.

Dickinson found that bees have an incredibly complex wing beat. The wing sweeps back in a ninety-degree arc, then flips over as it turns, all this happening astonishing 230 times a second. Like the rotation of a propeller, this generates more lift than the ordinary wing beats of larger insects.

Now, another triumph for Dickinson: the answer to the fly-swatting conundrum. High-resolution, high-speed digital imaging of fruit flies faced with a looming swatter (actually a 14-centimeter-diameter black disk, dropping at a 50-degree angle toward a fly standing at the center of small platform) show that no matter what position a fly is in when the threat appears–grooming, feeding, walking, or what have you–the appearance of the disk from any direction causes the fly to quickly–within about 100 milliseconds–adjust its legs into the optimal position to allow it to hop out of the way in the opposite direction.

It’s an amazingly quick and amazingly sophisticated transformation of sensory information to motor response, and it’s saved the lives of innumerable flies over the centuries.

Practical benefits from Dickenson’s research into insects’ aerial abilities include the possibilities of tiny flying robots for surveillance or search-and-rescue use, improved propellers, or more stable aircraft.

But here’s some advice you can use here and now: “It is best,” says Dickinson, “not to swat at the fly’s starting position, but rather to aim a bit forward of that to anticipate where the fly is going to jump when it first sees your swatter.”

See? Told you he was a genius.

Permanent link to this article: https://edwardwillett.com/2008/09/why-flies-are-so-hard-to-swat/


    • Scrawney John Smith on September 4, 2008 at 4:09 am
    • Reply

    There’s nothing more effective than Raid!

    • Janet on September 2, 2008 at 1:31 pm
    • Reply

    That is a very interesting article. But it proves yet again that even the experts don’t know everything.

    For those of us with pitiful hand/eye coordination, killing flies can still be a very easy thing. Like the proverbial frog in the pot, flies do not react to slow change. Position the flyswatter over a resting fly very slowly until it’s just a few inches away. Then a sudden swat, and the fly is history. They just can’t fly fast enough to get out of the swatter’s airspace in time.

    This is my contribution to the betterment of humanity for the day.

    • Ken McConnell on September 2, 2008 at 12:54 pm
    • Reply

    This is one of those life lessons that my father taught me when I was a kid. How to kill a fly with your hands.

    Basically, if you sweep up with your hands when you bring them together slightly above the fly’s position, you will usually get it.

    Interesting. I like to watch my cat kill flies with just his paws and mouth. I imagine that cats instinctively know to aim high. We humans have to be taught this.

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