Scientists combine evolution, physics, and robotics to decode insect flight

Different insects flap their wings in different manners. Understanding the variations between these modes of flight may help scientists design better and more efficient flying robots in the future. However, decoding insect flight is not as easy as it sounds.

Winged insects have been around for nearly 400 million years, and the evolution of flight in different insect species influences things like how insects flap their wings, what makes some insects highly maneuverable, and how their flight muscles work. A new study has used a mix of evolutionary analysis and robotic model wings to better understand how different flight modes operate.

Insects are the most skilled flyers

There are organisms other than insects that can fly. Scientists can also take inspiration from them, so what makes insect flight so special?

“From a basic biology perspective, flight has evolved four times in the history of animals (pterosaurs, birds, bats, and insects), but flight in insects is arguably the most successful in that it has been around for the longest (hundreds of millions of years) and has led to the largest number of species. So it is one of the prime examples of a key innovation in evolution,” Simon Sponberg, a professor of physics and biological sciences at Georgia Tech, told Ars Technica.

Sponberg and his team’s study sheds light on the evolution of flight in different insect groups. Using the findings from their research, they also modeled robots that mimicked two different flight modes.

Not all insects fly the same way

There are some insects that fly synchronously, meaning their wings on both sides flap together and in a coordinated manner. Others demonstrate asynchronous flight, in which each wing operates independently. A big difference between these two modes is that in synchronous flight, the nervous system of an insect has complete control over the wings’ motion.

The insects can command their muscles to beat on each wingstroke with their brains, just like you or I do when we command our leg muscles to move with each step. That is what the very first flying insects likely did, as it’s common in many groups of insects today, including moths, cockroaches, and others.

In asynchronous flight, the wings flap much faster than the insect’s brain can control. This is possible because of a special delayed stretch activation property in the flight muscles. When they get pulled on by other muscles and the wing, they automatically pull back, which happens faster than they can respond to a brain signal. “In this case, the brain basically says 'go' and the muscles themselves take over, and because of this special property, they vibrate really quickly. This is how you can get a muscle to beat wings at something like 800 times a second or more as you would see in a mosquito,” Sponberg said.

Insects switch flight modes as they evolve

The current study focuses on asynchronous flight because the insects that fly in this manner perform ultrafast wingbeats and are more maneuverable. These ultrafast wingbeats are present in at least four major insect groups, including mosquitos, bees, beetles, and true bugs or hemipterans. Since these groups are spread out on the evolutionary family tree, people had for a long time assumed that this strategy had evolved separately in each of those groups.

“We show that the asynchronous ultrafast flight in these four groups probably shared one evolutionary origin—they had one common ancestor that was ultrafast rather than each evolving independently. This means that all the slower flapping groups of insects in between those groups, like moths and butterflies, would have also had this common ancestor,” Sponberg told Ars Technica. However, throughout evolution, moths and butterflies somehow reverted to the slower, synchronous neural command.

“Our theory is that this type of ultrafast muscle was lost in those groups. Within some of these groups of insects, there have then been repeated transitions back and forth between the two flight modes. We believe they still have a remnant of the stretch activation present in the system,” Sponberg added. These findings also hint that there’s the possibility of a single insect species having properties of both synchronous and asynchronous flight.

Robots mimic evolutionary transitions

The researchers think that insect flight can be better understood in terms of oscillators. A person can keep a pendulum oscillating either by repeatedly pushing it or by placing the pendulum in a system where it is automatically pushed or pulled. Regardless of your method, the pendulum will take roughly the same time to complete one oscillation.

“A pendulum oscillates this way because of the force of gravity pulling on the pendulum and not any sort of predetermined pattern. Asynchronous insects also generate flapping wing motion based on the time history of the wing motion just a little time before. This process is called feedback-generated oscillations,” Nicholas Gravish, one of the study authors and a professor of mechanical and aerospace engineering at UC San Diego, told Ars Technica.

To further examine these two patterns of wing beats, the researchers developed two flapping robots. The first one was modeled on hawkmoths (Manduca sexta). These insects fly in a synchronous fashion, but the robot also has the necessary tools to perform asynchronous flight. The second robot was modeled after RoboBee, an autonomous flying robot developed by Harvard University researchers.

The RoboBee model had a body size similar to real hawk moths. It and other current flapping wing robots just follow the pattern of motion that is sent to them by a computer or microcontroller—much like synchronous flight. The first robot, on the other hand, was smaller in size compared to the real moths. It could perform asynchronous flight with stretch-generated wingbeats when placed in a liquid medium, mimicking the flight of small insects in the air.

The smaller robot could also switch to synchronous flight when connected to a controller that generated sinusoidal or periodic signals. “This paper includes the first insect-scale robot that uses this asynchronous, self-excited strategy for generating wingbeats, and it can even switch back and forth between the two modes,” Sponberg said.

The researchers wanted to engineer stretch-generated activation into the RoboBee model to enable asynchronous flight there, as well. In order to do this, they used a special fiber-optic sensor to measure how fast its wings moved. This speed data was uploaded in a computer program similar to the one that controlled wing flapping in the first model. This enabled a real-time feedback loop between wing velocity and actuator voltage, resulting in the generation of asynchronous wingbeats in the RoboBee.

This experiment suggests a simple mechanism for how the transition between the two styles of flight could occur and suggests that it was probably very easy for insect species to transition back and forth between the different flight modes.

Asynchronous flight and better robots

The current study suggests that the two flight strategies can actually be two regimes of the same system. This means that it could be much easier to switch between strategies in the same robot—and may mean that insects can switch, as well.

Beyond its implications for biology, this may also have some practical uses. For example, it could lead to the development of robots that could automatically stop and restart upon encountering an obstacle, or automatically adjust the frequency when perturbed.

“We also want to explore what the control and maneuverability tradeoffs are for using the two different strategies of flight. Faster is not always better for control, and we want to know what advantages and disadvantages each flight strategy provides. This is also really important for thinking about new robot designs, especially because we can now build flapping robots that are either synchronous or asynchronous or even switch between the two,” Sponberg said.

Additionally, the researchers are interested in looking more deeply within the different ultrafast flapping groups. Some of these groups (like flies) exclusively use asynchronous flight, while others, like the true bugs, have switched back and forth many times in their evolutionary history. The authors hope that their model might give some predictions about which groups might be able to switch more easily.

Nature, 2023. DOI: 10.1038/s41586-023-06606-3 (About DOIs)