Ever wondered how a mosquito can flap their wings fast enough to produce that high-pitched whine? Some insects are capable of beating their wings up to 1000 times a second. This remarkable ability is due to a specialised form of muscle known as ‘asynchronous’ flight muscle. Instead of being triggered to contract directly by individual signals from the brain – like the muscle contractions of birds and slower flapping insects – the high frequency contractions of asynchronous muscle are triggered by the initial downbeat muscle contraction. In turn, this stretches the upbeat muscle to drive the upward movement of the wing, starting a self-perpetuating cycle of rapid muscular contractions that are independent of input from the brain. Scientists believe that this kind of asynchronous muscle evolved from regular flight muscle that is controlled directly by nerve signals and that the two muscle types are distinct from one another. However, researchers at the Georgia Institute of Technology, USA, and the University of California, San Diego, USA, led by Simon Sponberg disagreed. They test this assumption and show in their new paper that fast asynchronous and slow regular flight muscle might be two versions of the same underlying flight muscle architecture.
The researchers initially examined the evolutionary history of all flying insects to see how many times asynchronous flight muscle evolved. They demonstrated that the most likely scenario is that this muscle evolved once but at some points in the history reverted back to the regular form of flight. One such reversion likely occurred in the hawkmoth species Manduca sexta. When the team looked closer at the hawkmoth's flight muscle, which is controlled directly by slower brain signals leading to slower wing beats, they found it retains the physiological properties of the fast-beating asynchronous flight muscle. This suggests that although hawkmoths reverted from the asynchronous muscle back to controlling each wingbeat directly, the physiological scaffolding that allows the muscle to be stretch activated has stuck around, even though it is now redundant. This transition from asynchronous flight muscle back to regular flight muscle, and vice versa, raises the question: what does flight look like when an organism is in between the two types of muscle? Showing that this muscle transition still allows for smooth flight is essential because otherwise organisms with ‘in between’ muscle types would not have been able to fly, effectively grounding the evolutionary argument. To show that this transition was possible, the researchers employed two techniques.
Firstly, they developed a computational simulation of their hawkmoths that contained all the features of the insects’ muscle and body movements during flight. This allowed them to show that given the right movement and muscle inputs – such as rate of flapping and the correct muscle structure – simulated insects with transitioning muscle types could still fly. To prove this, they modified a dragonfly-sized robot to be able to transition mechanically between asynchronous and regular flight modes. Using this robot, they were also able to achieve a smooth flight while switching between the asynchronous muscle powered flight and flight powered by regular muscle. The robot even remained airborne at the transition when the muscle contractions stopped being stretch activated and switched to being directly controlled by signals from the brain. Taken together, the results suggest that the two types of muscle are not so different after all, changing our understanding of evolution of flight in insects and opening up the possibility of building robots that better mimic insect flight muscle.
