Gut first

Going with your gut is automatic for caterpillars. When crawling, a caterpillar’s gut advances an entire step forward before the outside of its body catches up, new research reveals.

The peculiar discovery should help scientists design better soft-bodied robots that can search tight places such as mine shafts, the nooks behind walls or the rubble of a fallen building.

“Exposing the secrets of animal locomotion provides the biological inspiration for engineers,” says Robert Full of the University of California, Berkeley, who was not involved with the research. “This is the beginning of being able to create incredible and effective robots that will serve us well.”

When most four-legged creatures (and two-legged humans) run, the gut passively slides back and forth. But in caterpillars, the gut is actively propelled — or perhaps pulled — forward, says study coauthor Michael Simon of Tufts University in Medford, Mass. On the outside, a caterpillar crawls by lifting its hind legs and moving them forward, then the next legs and the next legs, until the front of the body finally moves. But inside, the whole gut is moving forward with the very first step, ahead of the rest of the body, Simon and his colleagues report in an upcoming issue of Current Biology.

“Nothing like this has been described,” says functional morphologist Dennis Bramble of the University of Utah in Salt Lake City, who was not involved in the research. “It’s an entirely unexpected phenomenon.”

Unlike creatures with skeletons and proper circulatory systems, caterpillars are basically long, soft eating machines, says Simon. The caterpillar gut is anchored to its outer body at the head, the back end, and to some of the porelike openings along the animal’s side that allow for gas exchange.

The researchers thought caterpillar guts would slosh with each step. Instead, by shooting high-intensity X-rays at a hawkmoth caterpillar (Manduca sexta, a.k.a. the tobacco hornworm) while it crawled on a treadmill, they documented a very consistent wavelike motion. The gut was moving forward ahead of the body and then falling back, like a piston.

Adding a bit of glitter to the caterpillar’s food confirmed the discovery. The tiny flecks of metal also moved back and forth as the animal walked. And when the team looked at very young caterpillars, which are translucent, the gut moved the same way it did in the older animals.

It isn’t clear if the gut is stretching itself forward and then moving back to the rest position, or if it is almost at rest when elongated, and then compressed, says Simon. It also isn’t clear why evolution might favor such a gut, which is so heavy that it actually shifts the caterpillar’s center of gravity forward before its exterior moves. Most caterpillars spend little time horizontal — they crawl upside down, on the undersides of leaves, and up and down tall stems. Perhaps the shifting gut is pertinent to this “gravity agnostic” lifestyle, Simon says.

Either way, the new work is definitely pertinent for building soft robots, says Satyandra Gupta, director of the robotics center at the University of Maryland in College Park.

“People have made soft-bodied robots, but there’s a lot of room for improvement,” Gupta says. “They aren’t as capable as things in nature.”