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Flies are evasive buzzing machines that make it nearly impossible to swat. Luckily, science has some explanation to help you predict their next move.

Hosted by: Stefan Chin

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Go to to learn how you can take your STEM skills to the next level! [ ♪ intro ♪ ] Swatting flies is hard work, because theyy don’t make it easy for us. They’re evasive machines that can flee in any direction without tipping off their predator, and they can do it fast.

Good luck getting one of them! But the good news is you don’t need luck to understand their sneaky ways if you have science. Here are some experimentally based ins and outs of flies that might help explain why it’s so hard to nab them.

First of all, flies already have a leg up on us…well, four more legs, really. And that’s just one of their advantages. In this battle to their death, flies bring three times the legs to the table.

Having six legs allows flies to balance themselves on their four outside legs and use the inside ones to get themselves ready to launch into the air at the best angle. For example, if they angle their two middle legs back toward their hind legs, they have leverage to launch forward. If they angle those legs forward toward the front, they have leverage to launch backward.

These legs give them that extra umph to aim where they plan to go. And they can do it before giving away the element of surprise. That’s because those middle legs are tucked under the fly’s torso, so it’s harder for you to see them until you’re right on top of the fly.

And by that point, they’ve seen you coming — because flies can also process what they see ten times faster than us. They have what’s called a higher critical flicker fusion frequency. It’s how fast you have to flicker a light on and off before an animal sees the light as always being on.

It’s kind of like how videos work: They’re a series of pictures going by so fast we interpret them as moving. Since a fly’s frequency is faster than ours, they’re getting more visual information every second, which might help them dodge your attacks. Flies also use head movements to slow down what they see and respond with a moment’s notice.

For instance, by moving their head along with the motion of your flyswatter, they can see it coming more steadily even when it’s happening quickly. Flies’ visual tricks might also help them keep their vision from blurring while they fly — because they can take off pretty fast. And in those speedy conditions, they need to keep their balance, too.

To do that, they have a special body part called halteres. Halteres are the shriveled-up evolutionary remnants of what used to be another set of wings. But instead of adding lift, they add balance.

It’s like how you might throw your arms out to help level yourself when you lose your balance. But with halteres, flies basically always have those arms out. The good news is that not all flies have these structures, so maybe we still stand a chance against some of them.

Because a fly that’s getting its bearings might be an easier target than one that’s stable and zooming confidently through the air. And I guess all those tools that humans have made over the years help, too. Now, overall, flies aren't like this by accident: They've evolved to be super evasive.

Throughout history, most of the slower flies have been eaten by speedy predators like birds, and only the most evasive insects got to reproduce. So, you should feel especially talented if you do manage to catch one! If you enjoy learning how we can use science to answer everyday questions, like why it’s tough to swat a fly, you would probably enjoy Brilliant’s course, Physics of the Everyday!

This interactive course explores physics concepts in familiar but unexpected places, like traffic, toilets… or even axe throwing. Brilliant is an online learning platform with courses t hat can help you cultivate your math and scientific thinking skills. And their courses are designed for people of all levels, so you can jump in at any point and work your way to mastery.

They’ve also redesigned courses like pre-algebra, mathematical fundamentals, and algorithm fundamentals from the ground up to be even more hands-on and interactive. You can sign up at to get 20% off the annual Premium subscription. Thanks again to Brilliant for sponsoring this episode, and thank you for watching! [ ♪ OUTRO ♪ ]