Tiny treadmills show how fruit flies walk

The runner on the treadmill wasn’t your typical gym rat. For one thing, he kept sprinting to the front of the treadmill and riding it to the back. For another, he had six legs.

The exerciser was a fruit fly — officially known as Drosophila melanogaster. For the first time, fruit flies can run forward on their own tiny treadmill surrounded by high-speed cameras. The results from those workouts, posted February 24 to bioRxiv.org, have already revealed insights about how bodies move.

“This opens a great opportunity to examine how the central nervous system of the fly controls different aspects of walking,” says Eugenia Chiappe, a systems neuroscientist at the Champalimaud Foundation in Lisbon, Portugal. And understanding how brains — of fruit flies, humans and other animals — pull this off is important, she says. “Our interaction with the environment, other animals and fellow human beings — crucial for our survival — depends on the ability of the brain to monitor and control movement.”

Made of simple and easy-to-get belts, pulleys and motors, the length of the fruit fly treadmill was over four times the body length of a fruit fly, which for flies in the study averaged about 2 millimeters. For the experiments, the flies’ wings were trimmed, preventing flight. A glass chamber, coated with slippery Rain-X to limit wall-walking, surrounded the treadmill. When the treadmill was on, the only floor available was moving.

Based on where the flies’ legs hit the treadmill, the researchers saw that flies moved as they normally do. Most of the time, the flies surged ahead, running quickly to the front of the treadmill and then standing as they rode it back to the end. When their abdomens nicked the back of the chamber, the startled flies would then sprint forward again. “They sprint and then stop, sprint and then stop,” says neural engineer Brandon Pratt at the University of Washington in Seattle.

The flies easily hit speeds of 50 millimeters, just under two inches, a second — the “fastest speed ever reported” for Drosophila moving on the ground, Pratt says. He’s confident the flies can go even faster, though the researchers didn’t systematically test this. What’s more, some flies turned out to be marathon-level athletes, easily handling treadmill sessions pushing an hour and a half.  

And, as the flies ran faster, their bodies moved higher, rising up on their tiptoes, or “tippy-tarsi,” as Pratt says. The flies could also move on treadmills modified to have two belts, forcing each side of their body to move at a different speed. This experiment showed that the flies’ middle legs are the legs that compensate for this odd arrangement, adjusting their step distances to keep the fly moving straight ahead. This middle-leg change in stepping is akin to how a fly might handle a sudden gust of wind from one side, turning into the wind by changing their strides.

One thing the flies wouldn’t do: Run backwards. “They like to run upriver,” Pratt says. 

The treadmill doesn’t allow scientists to record neural activity while the fly is moving, which would be a valuable bit of data, Chiappe says. Other systems allow more detailed glimpses into the brain of a moving fly, such as spheres that tethered flies can perch on and roll while holding still in space. But those systems don’t allow the fly to move as naturally.

In addition to producing insights on movement, the treadmill experiments sparked some friendly competition, Pratt says. “There was a point where we were like, ‘Can we find the most athletic fly?’ We started going down a rabbit hole,” he says. “We even did a side project where we did endurance training in flies. It got a little ridiculous.”