Robots developed by engineers at the University of California San Diego played a crucial role in a breakthrough study on the evolution of insect flight, published in the October 4, 2023 issue of the journal Nature. This study was the result of a six-year collaboration between roboticists at UC San Diego and biophysicists at the Georgia Institute of Technology.
The research focused on the evolution of two different modes of flight in insects. Most insects use their brains to activate their flight muscles for each wingstroke, similar to how we activate the muscles in our legs when we walk. This is called synchronous flight. However, some insects, like mosquitoes, are able to flap their wings without their nervous system commanding each wingstroke. Instead, the muscles of these insects automatically activate when they are stretched. This is known as asynchronous flight. Asynchronous flight is common in certain insect groups, allowing them to flap their wings at high speeds. For example, some mosquitoes can flap their wings over 800 times per second.
Previously, scientists believed that the four major insect groups-bees, flies, beetles, and true bugs (hemiptera)-all independently evolved asynchronous flight. However, the Georgia Tech team’s analysis suggests that asynchronous flight actually evolved together in a common ancestor. Subsequently, some groups of insects reverted back to synchronous flight, while others remained asynchronous.
This discovery led the researchers to investigate how insects like moths, which evolved from synchronous to asynchronous flight and then back to synchronous flight, are able to maintain both types of flight in their muscles. To answer this question, the researchers conducted insect, robot, and mathematical experiments. The Hawkmoth was chosen as the ideal specimen for this study because moths use synchronous flight, but their evolutionary record indicates ancestors with asynchronous flight.
The Georgia Tech team initially measured the Hawkmoth muscle to determine if there were any signatures of asynchrony. Through mechanical characterization, they found that the Hawkmoths still retained the physical characteristics of asynchronous flight muscles, even if they were not utilized.
To understand how an insect can have both synchronous and asynchronous properties and still fly, the researchers realized that using robots would allow them to conduct experiments that could not be performed on insects. For example, they equipped the robots with motors that could emulate combinations of asynchronous and synchronous muscles, and tested what transitions might have occurred during millions of years of flight evolution.
According to Nick Gravish, a professor of mechanical and aerospace engineering at UC San Diego, this research highlights the potential of robophysics, the practice of using robots to study the physics of living systems. He stated, “We were able to provide an understanding of how the transition between asynchronous and synchronous flight could occur. By building a flapping wing robot, we helped provide an answer to an evolutionary question in biology.”
James Lynch, one of the lead co-authors of the paper and a Ph.D. graduate from Gravish’s lab, explained that building robots with similar features to animals allows for a better understanding of their movement through the environment. He stated, “If you’re trying to understand how animals-or other things-move through their environment, it is sometimes easier to build a robot that has similar features to these things and moves through the same environment.”
Brett Aiello, one of the co-first authors and an assistant professor of biology at Seton Hill University, emphasized that one of the major findings of this research is that transitions between synchronous and asynchronous flight occur in both directions, and there is only one independent origin of asynchronous muscle. He said, “From that one independent origin, multiple revisions back to synchrony have occurred.”
The research also involved building robo-physical models of insects. Lynch and co-first author Jeff Gau, a Ph.D. student at Georgia Tech, studied moths and measured their muscle activity during flight. They then created a mathematical model of the moth’s wing flapping movements. Lynch brought this model back to UC San Diego and translated it into commands and control algorithms for a robot that mimicked a moth wing. The robots built were larger than moths, allowing for easier observation in fluid physics experiments.
Two robots were created for the study: a large flapper robot modeled after a moth, deployed in water, and a smaller flapper robot modeled after Harvard’s robo bee, operating in air. These robots helped researchers test how an insect could transition from synchronous to asynchronous flight. The experiments showed that under certain circumstances, an insect could gradually and smoothly transition between the two modes of flight.
James Lynch faced several challenges during the research, including modeling the fluid flow around the robots and accurately representing the feedback property of insect muscles when stretched. Lynch simplified the model while maintaining accuracy and discovered the need to slow down the movements of the robots to maintain stability.
Future steps for the robotics aspect of this research include collaborating with material scientists to equip the robots with muscle-like materials. The findings of this study not only contribute to our understanding of insect flight evolution and biophysics but also have implications for robotics. Robots with asynchronous motors could adapt and respond rapidly to environmental changes, such as wind gusts or wing collisions. The research could also assist in the design of improved robots with flapping wings.
In summary, the use of robots in this study provided insights into the evolutionary transitions between synchronous and asynchronous flight in insects. By building robotic models of insects, researchers were able to perform experiments and uncover key findings about the evolution and biophysics of flight.