Solving the thoracic inverse problem in the fruit fly

Journal article, 2023

In many insect species, the thoracic exoskeletal structure plays a crucial role in enabling flight. In the dipteran indirect flight mechanism, thoracic cuticle acts as a transmission link between the flight muscles and the wings, and it is often thought to act as an elastic modulator: improving flight motor efficiency thorough linear or nonlinear resonance. But peering closely into the drivetrain of tiny insects is experimentally difficult, and the nature of this elastic modulation is unclear. Here, we present a new inverse-problem methodology to surmount this difficulty. In a data synthesis process, we integrate experimentally-observableliterature-reported rigid-wing aerodynamic and musculoskeletal data into a planar oscillator model for the fruit fly Drosophila melanogaster, and use this integrated dataset data to identify several surprising properties of the fly's thorax. We find that fruit flies likely have an energetic need for flight motor resonance: absolute power savings due to flight motor elasticity range from 0-30% across literature-reported datasets, averaging  average 16%. However, in all cases, the intrinsic high effective stiffness of the active asynchronous flight muscles accounts for all the elasticity elastic energy storage required by the wingbeat. The D. melanogaster flight motor should be considered as a system in which the wings are resonant with the elastic effects of the motor’s asynchronous musculature, and not with the elastic effects of the thoracic exoskeleton. We discover also a fundamental link betweenthat the D. melanogaster wingbeat kinematics and musculature dynamics: wingbeat kinematics areshow subtle adaptions adapted to that ensure that wingbeat load requirements match musculature load outputmuscular forcing capability. Together, these newly-identified properties lead suggestto a novel conceptual model of the fruit fly's flight motor: a structure that is resonant due to muscular elasticity, and is thereby intensely concerned with ensuring that the primary flight muscles are operating efficiently. Our inverse-problem methodology sheds new light on the complex behaviour of these tiny flight motors, and provides avenues for further studies in a range of other insect species.