Recently on a moon-lit night, my granddaughter said, “We’ve got deer.”

Sure enough, out the kitchen window we could see two deer feeding in our yard.

“They know when it is safe,” I said. “Just like house flies.”

I didn’t need to explain – everyone knew what I meant. Hunting season? Deer will hide. Pick up a fly swatter? Flies disappear.

We were recently plagued by a particularly annoying housefly, the kind that sounds like a miniature fighter jet, and witnessed its self-preservation mode. Picked up a swatter (physical or electronic), the fly disappeared. Set it down, the fly was back.

I’ve seen this happen many times in my life, but never checked to see if it just seems like they know you’ve picked up an anti-fly device or if they actually know.


I went looking for data and could find none. There was lots of discussion and many people agreeing that flies disappear when you pick up a swatter, but there were no answers. However, there is good data on why flies are so darn hard to swat.

Throughout history, it has been a thing of wonder that flies can avoid a rolled up newspaper, a folded magazine, a house slipper, or a store-bought swatter with such ease.

It wasn’t until 2008 that the secret of their annoying success was discovered. Michael Dickinson, a professor of bio-engineering at the California Institute of Technology, published a paper called—wait for it—Visually Mediated Motor Planning in the Escape Response of Drosophila. (Drosophila is the scientific name for fruit flies.)

Dickinson’s team took high-speed, high-resolution video of fruit flies avoiding a swat.

First thing, flies can see nearly 360 degrees, so there is no angle that gives you an advantage. Think that swatting from behind improves your chances? Wrong.

What Dickinson discovered is that flies are excellent at positioning their legs and bodies so they can jump in any direction. And they can position their bodies and jump at speeds beyond our ability to swat.


In the paper, Dickinson says, “… we found that approximately 200 ms before takeoff, flies begin a series of postural adjustments that determine the direction of their escape. These movements position their center of mass so that leg extension will push them away from the expanding visual stimulus.”

In other words, in about a fifth of a second, a fly can position its body and legs for a jumping takeoff in any direction.

It’s a fascinating study, and the report includes six slow-motion video segments of flies escaping. Among them are flies preparing for a front looming threat, as well as a right, left, and rear looming threat.

The bottom line is, flies are quicker than we are. Not a surprise, cause otherwise we’d flatten them every time. Nonetheless, it’s interesting to see how they plan and execute their escapes.

As to how flies know we’ve picked up a swatter, at this point, magic is as good an answer as any.

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