How Did the Caterpillar Cross the Road?

JULY 27, 2010

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Scientists studied the Manduca sexta caterpillar under x-ray and found that when caterpillars walk, their guts move forward first, followed by the rest of their bodies in a rippling movement. (Courtesy National Public Radio)

Researchers have discovered that at least one species of caterpillar precedes each step with a thrust of its gut. The finding, based on experiments carried out at the U.S. Department of Energy’s Advanced Photon Source at Argonne National Laboratory points to an entirely new mode of animal locomotion and could lead researchers to develop new robotic tools for exploration and medicine.

From Cell Press Online: Most of us have surely seen the slow and gravity-defying crawl of a caterpillar, with that wave of motion that passes over their elongated and flexible bodies. But it turns out that inside those crawlers, there's a completely different motion going on: their rather primitive guts slide forward before anything else moves at all.

The discovery, reported online on July 22 in Current Biology, a Cell Press publication, shows that caterpillars make their way through the world using a form of legged locomotion unlike any described before. In addition to expanding scientists' understanding of crawling, the researchers behind the discovery say that the new insights are finding their way into designs for soft-bodied robots of the future.

"From the outside, a wave progresses from the back of the caterpillar forward, and it looks like it all moves along with the wave," said Michael Simon of Tufts University. "But the gut moves around inside the body. It's a strange decoupled movement."

Simon, along with Tufts' Barry Trimmer and their colleagues, made the discovery completely by accident. Their interest is in neurosensory systems and the integration of sensory information to produce movement. To learn more about how caterpillars move, the researchers took some young hawkmoths to Argonne National Laboratory, where they could use sophisticated X-ray technology to see what was happening inside the caterpillars. They were expecting to find fluid sloshing around, Simon said. But instead, they saw what looked like the caterpillars' guts moving independently of the rest of their bodies. (Imagine for a moment your internal organs lurching upward on their own.)

The researchers characterized the crawling motion further using both X-ray and visible-light videos. Those videos showed that, at the start of each crawl, the gut in the insects' midbody segments moved in advance of the body wall and before the attached limbs, known as prolegs, swung. "The midgut typically advanced an entire step forward before the body wall caught up," the researchers report. In fact, the gut lurches forward and then falls back in what they describe as a pistoning motion.

There have been previous examples of internal tissue movements in mammals and birds, but those have always been the result of simple inertia. For example, Simon said, the livers of horses can slide back and forth as the animals gallop along.

"The unusual phenomenon of visceral-locomotory pistoning that we describe here is not generated by cyclic inertial forces from the locomotion itself, as in previous reports," the researchers write. In fact, most caterpillars move so slowly that they can stop and restart during any part of their crawl cycle without major changes in the movement of other parts of their bodies.

The caterpillar's anatomy is likely key. If you were to open a caterpillar up, you would see what is essentially an open bag lined with muscle, Simon explained. Their digestive system, a fairly simple tube running from the mouth to the anus, is suspended inside. And unlike what you'd find inside an earthworm, there are no walls separating one segment of their bodies from the next. That leaves the gut free to move in what the researchers refer to as "a two-body system—the container and the contained."

It's not yet clear whether this sliding gut movement has advantages for the caterpillars, the researchers say, but it certainly might. Because the young hawkmoths are all about eating and growing, it could be handy to free the gut from the disruptions of crawling.

The researchers think that what they've found in the hawkmoths will apply to other caterpillars and perhaps a few other creatures, including leeches. And they say that the findings are already contributing to efforts in their laboratory to design and develop robots made from soft materials, which might be better equipped than your average robot to squeeze into tight spaces or, like caterpillars, be "gravity agnostic." Simon says a free-floating "gut" might give such robots some very useful cargo room.

From the New York Times: For caterpillars, what you see on the outside as they crawl is not necessarily what is going on inside. Shaped something like an accordion, the tobacco hornworm caterpillar moves one segment at a time. First, the back legs on the last segment step forward, then the second-to-last segment and so on until finally the front segment with its head moves forward. Researchers at Tufts University and Virginia Tech were curious about the interior biomechanics of this form of movement, so they stuck the caterpillars on a treadmill [at the X-ray Operations and Research 32-ID x-ray beamline at the U.S. Department of Energy’s Advanced Photon Source]… at Argonne National Laboratory.

Read the entire New York Times article, “New Insight into a Caterpillar’s Crawl,” at

From National Public Radio online: The question isn't why did the caterpillar cross the road but how? Researchers from Tufts University have discovered that at least one species of caterpillar precedes each step with a thrust of its gut. The finding points to an entirely new mode of animal locomotion and could lead researchers to develop new robotic tools for exploration and medicine.

Caterpillars don't have a bone in their body. They move by squeezing muscles in sequence in an undulating wave motion. It is easy enough to observe from the outside, but Michael Simon, then a graduate student at Tufts University wanted to know what was happening on the inside. Simon decided he needed to X-ray a caterpillar as it crawled.

That isn't as easy as it sounds. Because caterpillars don't have bones, they can't be x-rayed by conventional machines. So Simon and his group took the caterpillars to the Advanced Photon Source. They also brought a tiny, custom-built caterpillar treadmill.

"We put our caterpillar on top that treadmill in this big, shiny star trek room,” Simon says. Then came the really tricky part. “You'd be very surprised how difficult it is to make a caterpillar crawl when it doesn't want to."

Read the entire National Public Radio story, download a podcast of the interview, and watch a video abstract of the Current Biology journal article at:

From the Boston Globe: Inspiration for a new generation of soft-bodied robots that could burrow into inaccessible spots for everything from search-and-rescue to medical applications comes from a most unlikely place: the “moth closet’’ at Tufts University.

A paper published online in Current Biology described the use of powerful X-rays of a caterpillar crawling on a treadmill to reveal that, bizarrely, a caterpillar’s innards thrust forward before the rest of its body when it crawls, perhaps helping it to move.

Read the Boston Globe article, “Studying caterpillars to design robots,” at

See: Michael A. Simon1*, William A. Woods1, Yevgeniy V. Serebrenik1, Sharotka M. Simon1, Linnea I. van Griethuijsen1, John J. Socha2, Wah-Keat Lee3, and Barry A. Trimmer1, “Visceral-Locomotory Pistoning in Crawling Caterpillars,” Curr. Biol. (22 July 2010) Published online ahead of issue. DOI:10.1016/j.cub.2010.06.059

Author affiliations: 1Tufts University, 2Virginia Tech, 3Argonne National Laboratory

Correspondence: *

M.A.S., W.A.W., Y.V.S., S.M.S., and L.I.v.G. were funded by a National Science Foundation grant to B.A.T. (IOS 0718537). Use of the APS was supported by the DOE-BES, under Contract No. DE-AC02-06CH11357.

The Advanced Photon Source at Argonne National Laboratory is one of five national synchrotron radiation light sources supported by the U.S. Department of Energy’s Office of Science, Office of Basic Energy Sciences (DOE-BES). The APS is the source of the Western Hemisphere’s brightest high-energy x-ray beams for research in virtually every scientific discipline. More than 3,500 scientists representing universities, industry, and academic institutions from every U.S. state and several foreign nations visit the APS each year to carry out applied and basic research in support of the BES mission to understand, predict, and ultimately control matter and energy at the electronic, atomic, and molecular levels in order to provide the foundations for new energy technologies and to support DOE missions in energy, environment, and national security. To learn more about the Office of Basic Energy Sciences and its x-ray user facilities, visit

Argonne National Laboratory seeks solutions to pressing national problems in science and technology. The nation's first national laboratory, Argonne conducts leading-edge basic and applied scientific research in virtually every scientific discipline. Argonne researchers work closely with researchers from hundreds of companies, universities, and federal, state and municipal agencies to help them solve their specific problems, advance America's scientific leadership and prepare the nation for a better future. With employees from more than 60 nations, Argonne is managed by UChicago Argonne, LLC for the U.S. Department of Energy's Office of Science.