Evrim Yazgin
Cosmos science journalist
Fruit flies on treadmills are giving scientists insights into how insects walk in ways that previous, more invasive techniques could not.
Researchers want to understand how insects’ nervous systems respond to rapid changes underfoot. All animals must navigate potential hazards and changes in terrain, otherwise injury from falls would be likely (send this to your clumsy friend or relative).
Animals as diverse as flies, cockroaches, rats and humans all show similar ways of readjusting after a trip, for example.
Studying how insects adjust their walking will help scientists understand proprioception: how the body continually senses its articulation and movement.
These techniques have been helpful in evaluation and treatment of people, such as stroke patients, who have locomotive issues.
University of Washington researchers published in Current Biology their findings when fruit flies – Drosophila melanogaster – were put on specially-designed miniature treadmills.
Fruit flies are a good model for mapping neural locomotion control because they have a compact, fully mapped nervous system. Previous studies have also given scientists a suite of genetic tools to perform precise and specific manipulations of the fly’s nervous system.
Traditionally, researchers have studied insect locomotion either free walking or tethered.
Tethered insects have a small camera mounted on a stick attached to their backs. Unsurprisingly, this method is not the most comfortable for the insect, but an advantage of this approach is that it allows the fly’s movements on 3D surfaces to be studied.
“One disadvantage of studying locomotion in tethered flies is that their posture is constrained and normal ground reaction forces may be disrupted, which could affect walking kinematics,” the authors of the new study write.
Enter the Drosophila’s very own treadmill.
The researchers were able to track fly walking over long periods of time. Split-belt treadmills were used to investigate how the flies reacted to belts with different speeds on either side of the body.
Without the burden of a tethered camera, the flies were able to strut their stuff freely.
“At the extremes, flies on the treadmill were able to sustain walking at a max belt speed of 40 mm/s and surpassed an instantaneous walking velocity of 50 mm/s [about 0.18km/h], which is the fastest walking speed ever reported for Drosophila melanogaster,” the researchers say.
Split-belt treadmills had little effect on the coordination between legs. But the step distances of their middle legs changed a lot.
“The middle legs are ideally positioned to stably pivot the body of the fly about its centre of mass, like rowing a boat from its centre,” the authors write.
The researchers also tested what happened when the flies’ proprioception was impaired through genetic manipulation. They found the insects took fewer, larger steps. But coordination didn’t seem to be affected.
It’s possible that the nervous system compensated for lack of feedback, or because other neurons are more important for coordinated walking.
“These insights illustrate how treadmills fill an important gap between free-walking and tethered preparations for investigating neural and behavioural mechanisms for fly locomotion,” the study authors say.
