Testing the gearbox hypothesis for insect flight control.

Across Diptera, sustained flight relies on stretch-activated asynchronous flight muscles that generate multiple contraction cycles per motor neuron impulse, enabling wingbeat frequencies of ∼100-1000 Hz. These muscles attach to the thorax as indirect flight muscles, driving wing flapping through thoracic deformation, while smaller steering muscles modulate wing kinematics stroke-by-stroke. The wings articulate with the thorax through a wing hinge, where the radial stop (RS) and grooved pleural wing process (PWP) are proposed to act as a mechanical "gearbox" that modulates wingbeat amplitude during maneuvering. Supporting this hypothesis, previous studies observed that close RS-PWP interactions correlated with discrete wingbeat amplitude changes during fictive turns in tethered flies, although its functional significance in freely-flying insects remained untested. Here, we first examined the morphological diversity of the wing hinge across Diptera using scanning electron microscopy and found substantial variation in PWP morphology, suggesting that this putative gearbox is not highly conserved. We next tested gearbox function by bilaterally ablating the PWP in freely-flying houseflies (Musca domestica), released into an L-shaped arena requiring 90° yaw turns. Kinematic analyses revealed no significant differences between control and PWP-ablated flies in wing or body kinematics during yaw turns. Both groups differentially modulated wingbeat amplitude to generate yaw torques, indicating that PWP is inessential for wingbeat amplitude modulation and active neuromuscular control, rather than passive RS-PWP engagement, predominates in regulating wingbeat amplitude during maneuvers. Our findings challenge a central prediction of the gearbox hypothesis and highlight the robustness of the dipteran flight control system.