Elucidating the role of tergopleural muscles in fruit fly's pitch stability.*

Flapping flight is an inherently unstable form of locomotion that requires flying insects to constantly make slight adjustments to their wing motion in order to maintain stability. Previous studies have shown that, in fruit flies, the reflexes used to counteract these instabilities are fast, robust, and well-described by a proportional-integral (PI) controller model. To elucidate the neuromuscular underpinnings of this flight control reflex, we perform behavioral assays in which freely flying flies are simultaneously subjected to external mechanical torques and acute, targeted neuronal manipulations. By analyzing the 3D wing and body kinematics that flies produce in response to these paired perturbations, we assay the role of individual cells in the flight control reflex. Here we apply this methodology to investigate the motor neurons innervating a subset of the wing's control muscles, the tergopleural (tp) muscles. We find that inhibiting the tp muscles leads to an asymmetry in the body pitch response that cannot be captured by a PI model that only utilizes the forward sweep angle. This prompts us to consider an expanded control theory framework that captures the interplay between multiple wing degrees of freedom to explain the asymmetry observed.