Fruit flies on treadmills: Video shows how insects walk and balance

Have you ever wondered how fruit flies manage to move seamlessly within a complex and unpredictable world?

This seemingly trivial question is the focus of advanced research at the University of Washington School of Medicine in Seattle. The researchers used treadmills to study proprioception in fruit flies.

Miniature treadmills and fruit flies

A pivotal part of this research is the use of miniature treadmills, specifically designed for fruit flies. The study was led by Brandon G. Pratt, a recent physiology and biophysics PhD graduate and a National Science Foundation Graduate Research Fellow.

The treadmill setup is relatively simple. Pratt created it from inexpensive parts, improving on a prototype from Max Mauer, an alumnus from the same institution. The question is, why use a treadmill, and what’s the bigger picture?

Fruit flies’ movement and balance

Any walking creature, from insects to humans, needs to recognize and quickly adapt to unexpected changes underfoot. Failing to do so can result in a significant amount of stumbling and, possibly, a few unexpected crashes.

This is where the nervous system plays a vital role. It detects unexpected events and controls the body to regain balance, making navigation possible.

These balance issues are being studied at the UW Department of Physiology and Biophysics, under the supervision of associate professor John Tuthill, where Pratt carried out his doctoral research.

Pratt wasn’t alone in this endeavor. His colleagues in the lab, Su-Yee J. Lee and Grant M. Chou, contributed significantly to this intriguing project.

Proprioception: The unsung hero

Tuthill’s lab focuses on proprioception – the body’s ability to sense its movement and positioning. It’s an essential aspect of our coordination, with malfunctions causing difficulties in everyday tasks, like picking up a glass of water or simply walking.

In the lab, the researchers used their fruit fly treadmill experiment to study how proprioception controls the body when it becomes off kilter during movement.

This isn’t an easy task, as disruptions to proprioception can alter animal behaviors and confuse the examination of proprioception’s role in natural activities.

Treadmills: Not just for humans

For decades, treadmills have been effectively used to reignite the desire to walk in animals with perturbations to the nervous system.

Not just in fruit flies, but also in a variety of creatures like cockroaches, rodents, cats, and of course, humans. There’s even a variation, known as split-belt treadmills, that help researchers understand leg coordination when the legs on one side of the body move at a different speed than those on the other.

Inspired by these methods, Tuthill and his team designed miniature versions of these treadmills to study locomotion in fruit flies.

And why fruit flies? Because they’re a good model system. With a compact, fully mapped nervous system and the availability of genetic tools, they offer unique opportunities for manipulating the nervous system precisely.

Breakthroughs in fruit fly locomotion

By coercing flies to walk on the newly engineered treadmills, the investigators could track their movement in 3D over the long term. This allowed them to analyze walking at various speeds in flies with and without impaired proprioception.

The researchers found that the fruit flies walked in bursts on the mini treadmill. They sprinted to the front of the treadmill chamber, then rode the belt back. Roughly half their time was spent walking, and they sped up when the treadmill did.

Their body height even rose when they walked faster – a trait shared with humans and cockroaches. The highest walking speed ever reported for fruit flies, an impressive 50 millimeters per second, was achieved in these experiments.

The key to coordination

When the researchers genetically silenced neurons underlying proprioception and ran the insects on the treadmill, they noticed another interesting phenomenon.

The flies took fewer but larger steps. Yet, the coordination of their legs didn’t seem to be affected.

This suggested that other proprioceptive neurons might be key to coordinating walking, or the nervous system might have compensated for the lack of feedback.

Treadmills: A valuable research tool

The researchers from University of Washington School of Medicine in Seattle have found treadmills to be invaluable tools for investigating the neural and behavioral mechanisms of fly locomotion.

From their work, the experts have realized that treadmills bridge an important gap between free-walking and tethered preparations, enlightening the scientific community further on fly behavior.

In a display of intellectual generosity, the team has provided the designs of their miniature treadmill systems as a free, open source for fellow scientists.

The research was backed by numerous grants and awards, including a National Science Foundation Graduate Research Fellowship and a UW Innovation Award.

The study is published in the journal Current Biology.

Video Credit: B.G. Pratt et al/bioRxiv 2024

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