When a caterpillar crawls, its internal organs slide forward inside its body before its legs move. Does that matter? It does if you’re a caterpillar, but it also does if you’re a designer of soft-bodied robots. A team of researchers working at Massachusetts' Tufts University used an X-ray to observe large, opaque-bodied caterpillars, then backed up their findings by examining smaller, translucent caterpillars under a microscope. In both cases, it was observed that the caterpillar’s internal center of mass moved forward first, while its middle legs remained attached to the substrate. In a paper on their findings, the team wrote that the so-called gut-slide is “unlike any form of legged locomotion previously reported and represents a new feature in our emerging understanding of crawling.”

The scientists chose to use the caterpillars’ tracheae as a reference point, as the gas-filled tubes connect to both the body walls and the guts. It was observed that each “crawl” began with a step of the rear legs, followed by a wave of section-by-section contractions that moved the gut forward, followed by a catch-up step of the corresponding legs and body wall.

“This type of two-body mechanical system has never been seen before, and is probably unique to soft, squishy animals,” said team member Jake Socha, an assistant professor of engineering science and mechanics at Virginia Tech.

Gut-sliding locomotion could now find its way into soft-bodied robots, also known as softbots. These could include shape-changing search-and-rescue robots, zero-gravity space robots, and biocompatible medical robots that would minimize incidental tissue damage and discomfort.

The team’s research was recently published in the journal Current Biology.

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