One of the creepiest encounters I’ve ever had came 25 years ago, when I moved into my first house. Turning on the bathroom light early one morning, a gigantic centipede—with its blur of legs—ran up and over my foot without breaking stride. Now, even the sight of one sends shivers up my spine. Not so for Daniel Goldman, a biological physicist at the Georgia Institute of Technology (Georgia Tech) whose lab has just worked out how these invertebrates are so adept at scampering across feet, sand, soil, rocks—and even water. What’s more, he and his colleagues reported last month at the virtual meeting of the Society of Integrative and Comparative Biology that they have created a centipede robot that might one day scutter through farmers’ fields to take out troublesome weeds.
“Their results hammer [home] the point that flexibility [makes animals] capable of a broad range of behaviors,” says Jake Socha, a comparative biomechanist at Virginia Polytechnic Institute and State University (Virginia Tech) who was not involved with the work. The findings, he adds, reveal the previously unknown principles of centipede movement.
Though centipedes don’t typically have the 100 legs they are named for, they often have dozens of pairs, one per body segment. That makes them long as well as leggy—and capable of a variety of motions, says Matthew McHenry, a biomechanist at the University of California (UC), Irvine. The primitive arthropods have “speed, elegance, and efficiency that are the envy of engineers,” McHenry says. Despite that, our intuition about how they move is “often wrong,” Socha says.
Goldman had long been fascinated by centipedes, but analyzing their movements was nearly impossible, because they have too many body segments and legs to track. His interest was piqued when his postdoc, electrical engineer Yasemin Ozkan-Aydin, discovered the value of having numerous body segments and legs. When Ozkan-Aydin, now a robotics scientist at the University of Notre Dame, hooked two or three four-legged robots together, the longer machines could cross wider gaps and clamber over bigger obstacles, even outside on natural terrain, she and colleagues reported last year in Science Robotics. So Goldman’s lab went “all in” on centipede movement.
Another student, Georgia Tech undergraduate Eva Erickson, looked at how centipedes change the way they run. Many animals, including horses and people, alter their gait—the way their legs move relative to their body—as they increase their speed. But, using a sophisticated video tracking program called DeepLabCut, Erikson found the centipede Scolopocryptops sexspinosus instead changes its gait to match the challenges of its terrain.
Normally, S. sexspinousus’s legs move in a wave—like fingers drumming on a table. But sometimes, the direction of the wave changes. On flat surfaces, the wave starts with the last leg and travels headward. But when the going gets tough, the wave reverses, with the front leg moving first to establish a foothold, Erickson reported at the meeting. After that, each leg follows in kind, landing in the exact same spot as the previous leg. The taller the obstacles, the more likely the centipede is to adopt this so-called retrograde locomotion, Erickson said.
Should they wind up in the water, centipedes can also “swim” to rescue themselves. When Kelimar Diaz, a graduate student at Georgia Tech, investigated the behavior, she, too, found centipedes adjusted their movements to their environment. Videos reveal the species Lithobious forficatus starts to “swim” by flailing its legs. But then it quickly twists its body from side to side, at which point it moves forward (see video, below). “Somehow, the centipedes know that undulating their bodies generates the right forces,” Diaz reported at the meeting.
Another graduate student in the lab, Baxi Chong, has analyzed how separate waves in the legs and the bodies sync up. To predict which combination of body waves and stepping works best, he used a mathematical modeling technique first proposed by particle physicists. The model produced numerous combinations of leg and body waves, which he then tested in a centipede robot built solely for understanding how these animals move.
First, Chong programmed the robot to move its legs and body in sync. But he soon found the robot was faster when there was a lag between the two waves, he reported at the meeting. Further testing revealed some combinations made the robot move backward. These experiments, says UC Berkeley integrative biologist Robert Full, have “unraveled the secrets” of centipede coordination.
Subsequent work by Goldman’s group has found the robot works better if its legs are jointed and the body segments are pliable. “Being soft and squishy has lots of advantages both for animals and robots,” Socha says. “And animals that are long and skinny can take advantage of combinations of waves” to move, he adds.
Goldman’s latest robots are flexible, and they combine the fleet-footedness of a cockroach with the reach of a snake. The next step, he says, is to train them to do practical tasks, such as weed identification and eradication. Working with Philip Benfey, a plant development biologist at Duke University, Goldman plans to outfit his robots with artificial intelligence software that can single out individual weeds; the duo envisions robots that could then inject herbicide or use a laser or electrical discharge to kill them.
And if Ozkan-Aydin has her way, one day swarms of robotic centipedes will help humans with other tasks: planting and harvesting crops, moving goods, monitoring the environment, and even space exploration. That’s fine with me, as long as they stay off my feet.