Insect wings flap faster than their brain speed can command

Understanding the evolution of insect speed, particularly across various species, is no simple task. For example, mosquitoes, apart from being known nuisances, have always fascinated scientists for their rapid wingbeats. Astonishingly, mosquitoes can flap their wings over 800 times per second.

How can such a tiny creature achieve such speed, especially when the pace at which its muscles flap outpaces the speed at which its nervous system commands them to beat? This odd asynchronous beating, a phenomenon that seemed alien to many, is a key to understanding an evolutionary marvel.

Asynchronous flight drives insect speed

The principle of asynchronous beating means that there’s a detachment between the neural commands and muscle contractions. Interestingly, this is observed in only four distinct insect groups.

The assumption for years was that these groups evolved these unique wingbeats separately. However, a game-changing research collaboration between the Georgia Institute of Technology and the University of California, San Diego (UC San Diego) suggests otherwise. These high-frequency wingbeats, they discovered, originated from a single common ancestor.

As Jeff Gau, one of the lead authors of the study and a Ph.D. graduate from Georgia Tech, highlighted, their findings delve deep into evolutionary history, tracing back to how ancient insect muscles functioned around 400 million years ago.

“Our findings are pretty robust to all different experimental conditions,” said Gau. “We’re looking back 400 million years into how ancient insect muscles must have behaved from an evolutionary standpoint.”

Some insects are in sync

Moths, in contrast to mosquitoes, synchronize their flight muscles with every wing stroke. But, in an evolutionary twist, moths’ ancestors possessed asynchronous flight capability but lost it over time. The catch? Moths still have the capability for asynchronous muscle contractions.

To decode the evolutionary journey, scientists mapped flight strategies onto the two foundational ways physicists interpret oscillations. By integrating biophysical models with cutting-edge robotics, they showcased that these two strategies are essentially two facets of the same model.

A few evolutionary tweaks, and an insect can seamlessly transition between synchronous and asynchronous flight modes.

How size impacts insect speed

Size plays a crucial role. Many larger insects achieve synchronous flight, matching wing movement with nervous system pulses. But, for their smaller counterparts, this becomes a challenge. As they grow smaller, their wingbeats amplify, reaching speeds of up to 100 times per second.

According to Simon Sponberg, Dunn Family Early Career Associate Professor of Physics and Biological Sciences at Georgia Tech, there’s a natural speed barrier. If these tiny insects attempt to contract and relax their wings at such a pace, the wings would overlap and ultimately malfunction.

“As insects became smaller, their wingbeats increased to 100 times per second, and when you start getting up to that speed, there’s sort of an inherent speed limit where the muscle can’t contract and relax fast enough,” said Sponberg. “If they tried to contract and relax the wings, they’d start overlapping and then eventually lock up.”

Instead, smaller insects evolved a unique mechanism. Their nervous system dispatches a pulse of activity, priming the muscles for contraction, regardless of the need for a wingbeat. This efficient method ensures that wings flap at a remarkable speed without awaiting neural activation and relaxation for each motion.

New understanding of insect speed evolution

Historically, scientists believed that asynchronous flight was an evolutionary trait that insects stumbled upon independently to increase their speed. But recently released phylogenies, evolutionary family trees of species, painted a different picture.

Asynchrony didn’t emerge individually in separate evolutionary timelines. It sprouted once and then branched out. Over the ages, some insect groups thrived with their newfound speed and held onto it. At the same time, others reverted to synchronous flight.

Sponberg uses the analogy of oscillations to illustrate this concept. The principle of a dancing balloon at a car dealership, oscillating continuously, not by external force but due to a consistent air jet, mimics the way asynchronous flight functions in insects.

“If you’ve ever watched one of those dancing balloon guys at a car dealership, it goes up and collapses repeatedly,” Sponberg said. “What’s happening there is it’s oscillating, not because you’re poking it regularly, but you’re actually providing a continuous air jet in the bottom, which is a trade-off with the force of gravity.”

Robots mimicking nature

To deepen their understanding, the researchers at UC San Diego developed robots inspired by the moth’s flight mechanism. Two distinct robots were created: a larger flapper robot operating in water and a smaller flapper robot, modeled after Harvard’s Robobee, functioning in the air.

These robots weren’t mere scientific toys. They were crucial in proving that the models developed to explain flight transitions and insect speed held up in practical scenarios.

“You don’t need robotics to learn something about biology,” said Nick Gravish, an associate professor at UC San Diego. “But there’s something about building a bio-inspired robot that forces you to put yourself in the animal’s shoes.”

James Lynch, a Ph.D. graduate from UC San Diego and co-lead on the paper, emphasized the significance, noting that they had essentially built the first robot capable of mimicking evolutionary transitions in insect flight.

 “The physics of this much larger robot moving much more slowly are similar to those of an insect that’s a lot smaller and moving a lot faster,” said Lynch.

Interdisciplinary brilliance

Such discoveries were only attainable through the confluence of varied disciplines, including physics, evolutionary biology, and robotics. This interdisciplinary approach, as Brett Aiello from Seton Hill University noted, is pivotal in gaining profound insights into natural processes governing animal movement.

“One of the biggest evolutionary findings here is that these transitions are occurring in both directions and that instead of using multiple independent origins of asynchronous muscle, there’s actually only one,” said Aiello. “From that one independent origin, multiple revisions back to synchrony have occurred.”

In essence, the study is a testament to the magic that happens when different fields unite with a shared purpose. Through this collective effort, we now have a clearer understanding of the wonders of insect flight and the evolutionary dance that has shaped it.

The full research paper was published in Nature, titled, “Bridging two insect flight modes in evolution, physiology, and robophysics.”

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