Wing-flapping robot helps explain the evolution of insect flight

Some insects can flap their wings so rapidly that it’s impossible for instructions from their brains to entirely control the behaviour. Building tiny flapping robots has helped researchers shed light on how they evolved to do this.

If you flap your arms, each movement happens after your brain directs your arm muscles to contract and then relax. Something similar happens for many insects as they beat their wings. But for some, including mosquitoes, those brain signals and flapping are out of sync. After the initial signal to contract, the insects’ muscles undergo additional contract-relax cycles before they even receive another impulse from the brain. This so-called “asynchronous” flight allows them to flap their wings at exceptionally high rates.

Simon Sponberg at the Georgia Institute of Technology and his colleagues set out to understand the evolutionary history of this form of insect flight.

They began by gathering data on how many insects are capable of asynchronous flight. Then, they used an evolutionary tree of all insects to run computer simulations designed to assess how many times the behaviour evolved.

Four different groups of insects are currently capable of asynchronous flight, and researchers previously thought that each group evolved the ability independently. However, the new simulations suggested otherwise: they showed that, in 87 per cent of possible evolutionary scenarios, the trait only evolved once.

The simulations also suggested some modern insects that are incapable of asynchronous flight had ancestors that could fly this way. So the researchers decided to focus on one such species – the tobacco hawk moth (Manduca sexta).

When they measured the mechanical properties of the hawk moth’s muscles, including their stretchiness and how quickly they relaxed, the team found that the insect was not fully incapable of asynchronous flight. The reason hawk moths don’t fly this way is because their flight muscles’ impulse to contract in response to brain signals alone is stronger than their tendency to switch to an automatic, independent contraction cycle. “Instead of (asynchronous flight) being totally lost, it’s just turned way down,” says Sponberg.

This discovery motivated the researchers to build a robot capable of switching between the two types of flight when its flapping mechanisms are tuned in a similar way to the hawk moth’s muscles. Sponberg says their insect-sized “roboflapper” is the first to ever fly asynchronously, using self-excitation of its artificial muscles.

Douglas Syme at the University of Calgary in Canada says that scientists previously assumed it would be as difficult for an insect to switch between the two types of flight as turning a “piston engine into a jet engine”. But the new study suggests otherwise, he says, showing that it is more like turning “the engine of a pickup truck into the engine of a Ferrari – which are fundamentally the same, just tweaked for different applications”.

The discovery could shed light on why insects have transitioned between the two modes of flight as they have evolved, he says.

However, Robert Dudley at the University of California, Berkeley says the team’s evolutionary model was mostly based on the structure and microscopic properties of insect anatomy. He says the idea that asynchronous flight only evolved once could be made more convincing by cross-referencing it with measurements of wing flapping frequencies from insects in the field. But he thinks the study may still provide useful information for engineers building flying robots.