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MARY LOUISE KELLY, host:

You know the old joke: Why did the chicken cross the road? Well, scientists are considering a new twist: Why did the caterpillar cross the road? Or actually more like, how did the caterpillar cross the road? In this case, the answer is really surprising.

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.

Mr. MICHAEL SIMON (Researcher, Tufts University): 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.

Mr. 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.

Mr. 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.

Mr. 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.

Mr. 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.

Mr. 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.

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

BRUMFIEL: At the start of its step, the caterpillar was thrusting its entire digestive system towards the front of its body, and at the end, the gut slid back. The caterpillar was literally going with its gut.

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.

Professor MICHAEL LaBARBERA (Organismal Biology and Anatomy, Geophysical Sciences, University of Chicago): It's a whole different solution to the problem of locomotion.

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

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

BRUMFIEL: In fact, it's opened up a whole can of caterpillars. For one thing, scientists aren't quite sure how the gut thrust helps the caterpillar move. It might be changing the caterpillar's center of mass or stimulating its primitive circulatory system, or it may be that it's not directly about moving.

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

Mr. 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: It's more than just an academic question. The Tufts lab hopes to build a robotic caterpillar that could wiggle its way into hard-to-reach places. Further study is clearly needed, and that likely means more caterpillars will be getting a workout.

Geoff Brumfiel, NPR News.

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

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