When you have six legs, walking can potentially get pretty complicated. So many insects, including ants, simplify locomotion by moving their legs in two groups of three. It's called an alternating tripod gait: The front and back legs on the left side of the ant and the middle leg on the right side move together. While one leg trio contacts the ground, the other swings forward.

It's like having two really big feet, says Reinhard Blickhan, chair of the motion sciences department at Friedrich-Schiller University, Jena, who studies the locomotion of ants and other small animals. "They are very stable that way."

Okay, fine. But what about running? In a study in the Journal of Experimental Biology, Blickhan and his colleague Lars Reinhardt report new information about ant sprinting. Ants always keep at least three legs on the ground even at their fastest pace. Instead of leaving the ground, they speed up by taking longer, more frequent steps.

Scuttling Gait

It's a curious finding because most animals leave the ground completely during a running stride. Humans certainly do; it's called the "aerial phase" of our running stride. Most animals, whether on two- or four-legged, are like us in that respect, Blickhan says. One of the most famous experiments with early photography proved that horses leave the ground completely when they gallop.

But the more legs you have, the more likely you are to keep some of them on the ground, even when while running, Reinhardt says. It so happens that many of our many-legged creatures are very small.

"We now understand that grounded running is a gait that is very common among the small guys," Blickhan says, "and it allows [animals] to run without taking off." In addition to ants, cockroaches, spiders, and crabs, as well as some birds, also exhibit this scuttling gait when they run.

Run, Ants, Run

Understanding the way ants run required both a study of kinematics—the way the animals move through space—and of ground reaction forces (the pressure that the ants exert on the surfaces they traverse). The new paper is the first report of ground reaction force measurements for ants—specifically the wood ant.

Ground reaction forces are measured on force plates, which are devices that convert the mechanical energy of whatever traverses them (ants, in this case) to electrical signals. When Blickhan and Reinhardt tried to measure these forces in ants, they found that the existing technology was not up to the job. The plates used for studying larger insects, such as the cockroach, don't work for species weighing under a gram, and a single wood ant used in these experiments weighed around 20 milligrams. There are silicon-based devices that work in that force range, but they are brittle and "didn't survive more than for or five runs" with ants, Blickhan said.

To surmount these problems, the researchers designed their own force plate, using plastic shaped through stereolithography, the oldest form of 3D printing. They equipped the force plate with three semiconductor-based strain gauges, one per dimension. The result: A device both sensitive (recording within the micronewton range) and sturdy.

"They used some fantastic fabrication techniques to tune the force transducer to ant feet," Duke University biologist Sheila Patek, who studies the mechanics of motion in both ants and shrimp, tells PopMech via e-mail. "That is really a marvel of engineering." The researchers also used a high-resolution video camera to observe the ants' trajectories through space.

Extremely Agile

Keeping three legs on the ground at all times means that the ants can always turn and react to their terrain, the researchers say.

"Ants are crawling; they are always on the ground; they are extremely agile," Reindhardt says, "so they can perform quick turns, and this is only possible when they have legs on the ground"—you can't change direction, in the air, of course. And because ants sometimes walk up steep surfaces and even upside down, it behooves them not to let go, Blickhan adds.

Next, the researchers want to learn more about the neuromechanics of ant locomotion: what kinds of terrain information can ants glean through their legs? The research could also aid in the development of insect-inspired microrobots, Blickhan says.

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