Caterpillars move guts-first
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X-ray analysis shows the insects slide their guts ahead before moving the rest of their body in a unique form of motion that may inspire robot designs. (The article uses a photo of my old friend, the tobacco hornworm.)

The gravity-defying crawl of caterpillars, which has the guts sliding forward before the rest of the caterpillar moves, represents a unique "two-body" system of locomotion that has never been reported before for any other insect or animal, according to a new study.

In addition to shedding light on how organisms can crawl in three dimensional structures, such as tubes and hollow branches, the finding -- reported in the latest issue of Current Biology -- is also inspiring new designs for soft-bodied robots.

Humans, however, could never truly mimic a caterpillar crawl with our own bodies -- our guts aren't loose enough.

"One way to think about it is to imagine you're crawling on your hands and feet," lead author Michael Simon told Discovery News. "Normally, as you move forward, all of your internal tissues move forward with you. It makes sense, as it's all packed into your rib cage. But imagine if, instead, as you began to bring your back leg forward, your liver suddenly began to slide upward toward your neck."

"Quite impossible in humans, of course, because of how our organs are packed, attached to one another and to our skeleton, but the caterpillar gut is not so confined," added Simon, a Tufts University research assistant in the Biomimetic Devices Laboratory.

He and his team made the discovery by accident, while using sophisticated X-ray technology to see what happens inside caterpillars when they move. This was for a planned neurosensory experiment. The scientists expected to find fluid sloshing around inside the caterpillars.

Instead, they determined that at the start of each caterpillar crawl, the gut in the insect's mid-body segments moved in advance of the body wall and before the attached limbs, known as prolegs, swung.

"Basically the movements of the gut are independent of the surrounding body," Simon explained. "This is not to say that it is totally disconnected; it still is at both ends. But the body wall undergoes muscular contractions that move the body forward, whereas the gut changes shape and moves around within the body based only on the movements of the front and back of the animal."

The researchers predict leeches might also have such a sliding gut locomotion system. As Simon points out, "the leech gut already has considerable freedom of movement, since it must absorb such large blood meals."

The scientists theorize this form of movement benefits insect "eating machines" that spend much of their time feasting. In the case of caterpillars, their chief goals in life are to eat, grow and survive.

"Anything that would interfere with these goals would be a disadvantage," explained Simon. "If the gut were confined to individual segments, as in an earthworm (which is segmented), I would speculate that body movements would be transmitted through individual parts of the gut."

This, he said, might interfere with crawling, which is a forward-moving wave, and digestion, which involves backward-moving contractions.

Aside from promoting efficiency, the unusual gut-first, body-later movement may also improve the insect's balance, control and ability to grip. The technique is already being considered for soft-bodied robots with the ability to carry internal cargo, such as chemicals, electronics or even smaller robots.

"So, for example, if I wish to design a search and rescue robot that can morph its body shape as it winds through debris, as I'd like it to carry, say, a video camera and microphone to transmit information back, the way the electronics are carried within the robot may be dictated by, and play a role in, the control of the robot's movements," he said.

Susan Ernst, a Tufts professor of biology who did not work on the project, agreed that such robots and related research "could help scientists and engineers around the world navigate complex and even dangerous situations."

Simon says he's still surprised something so "commonplace and humble" as a caterpillar could exhibit such a "wealth of amazing phenomena" with so many important potential applications.