Gut Check: How Do Caterpillars Walk? Caterpillars don't have a bone in their body. They move by squeezing muscles in sequence in an undulating wave motion, but what exactly happens on the inside? Scientists wanted to find out, so they got a tiny, custom-built caterpillar treadmill.

Gut Check: How Do Caterpillars Walk?

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NPR's Geoff Brumfiel reports.

GEOFF BRUMFIEL: Michael Simon just finished his Ph.D. on caterpillar locomotion. A caterpillar has lots of legs but no bones.

MICHAEL SIMON: It's pretty much entirely made of soft materials, and that's very different from how we're constructed. We have muscles on our bones to help swing them back and forth. The caterpillar doesn't have that. So, it naturally brings up the question: How does the caterpillar crawl?

BRUMFIEL: Caterpillars contract their muscles from back to front in a wave motion as they walk. But Simon wanted to know exactly what was going on on the inside. So he and his group from Tufts University decided they needed to take some X-rays. Because caterpillars don't have bones, the group had to go to a special facility at Argonne National Laboratory in Illinois.

SIMON: We got the proper paperwork to transport insects into the state and we flew them by commercial jet.

BRUMFIEL: They also brought a tiny, custom-built caterpillar treadmill and placed it right in front of the powerful X-ray source.

SIMON: We put our caterpillar on top of that treadmill in this big, shiny "Star Trek" room, and then we'd close up the room, which involved red lights and Klaxons and voices announcing warnings and a big pneumatic door with magnetic locks.

BRUMFIEL: And then came the really tricky part.

SIMON: You'd be very surprised how difficult it is to make a caterpillar crawl when it doesn't want to.

BRUMFIEL: Seriously, I'm not making this up.

SIMON: Well, we've tried foods. We've tried smells. We've tried blowing on them. I don't want to give your listeners the wrong impression, but we do try stroking them gently with various objects to see if that encourages them.

BRUMFIEL: When they finally got that caterpillar crawling at full speed, they turned on the X-rays.

SIMON: And I was watching this monitor. We couldn't see the caterpillar, but we could see its X-ray. I'm watching the monitor and my adviser, Barry Trimmer, is watching it. And I say to Barry, did you see that? And he said, yeah, I saw that.

BRUMFIEL: What they saw was completely unexpected. Something on the inside of the caterpillar moved before each step.

SIMON: Eventually, we were able to establish that we were watching the gut moving back and forth.

BRUMFIEL: In a paper appearing in the journal Current Biology, Simon and his group make the case that this is an entirely new way to get around.

MICHAEL L: It's a whole different solution to the problem of locomotion.

BRUMFIEL: Michael LaBarbera studies invertebrate movement at the University of Chicago.

BARBERA: For geeks like me and, you know, a hundred of my closest friends, this is really deeply interesting.

BRUMFIEL: Michael Simon says the gut thrust might be a way of letting the caterpillar eat on the run.

SIMON: The whole point of the caterpillar is it's an eating machine - it eats and it grows. And anything that interferes with either of its functions is a disadvantage to the caterpillar.

BRUMFIEL: Geoff Brumfiel, NPR News.

LOUISE KELLY: Well, we're actually not making this story up. And if you don't believe us, go to, where you can see video of the caterpillars strutting their stuff on those mini-treadmills.

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