The Kinematic & Neuromuscular Basis of Drosophila Larval Escape

Escape behavior is the critical output of rapid sensorimotor processing in the brain that allows animals to sense danger and avoid it. The circuit structures and mechanisms that underlie escape are still under investigation. Drosophila larvae are an advantageous system for studying the neuromuscular circuitry of escape behavior. When threatened with harmful mechanical touch, heat, or light, larvae perform C-shaped bending and lateral rolling, followed by rapid forward crawling. The sensory input and neural circuitry that promotes escape in the larva have been extensively characterized, but we do not understand how bending and rolling motor programs are generated by the larval neuromuscular system. This work identifies the movement patterns, muscle activities, and motor circuit features that drive escape behavior. High-speed imaging approaches reveal that larvae select between four distinct, interchangeable patterns of escape rolling, and that each pattern consists of synchronous rotations of every segment as the larva rotates. Investigating electron microscopic reconstructions of premotor and motor neurons elucidates premotor to motor connectivity patterns that could underlie sequential muscle activity that circumnavigates the larva and propels synchronous rotation along the whole body. Volumetric Swept Confocally-Aligned Planar Excitation (SCAPE) microscopy uncovers that, unlike larval crawling, a well-studied form of larval locomotion that is driven by bilaterally symmetric peristaltic waves of muscle activity, the muscle activity during bending and rolling occurs in a circumferential sequence that is synchronous along the larva’s segments. Muscles neighboring the dorsal and ventral midlines of the larva demonstrate left-right symmetric activity during rolling, and ventral muscles appear to drive the propulsion. Shifts in magnitude of left-right symmetric activity in midline muscles allow the larva to transition from initial escape bending into escape rolling. Preliminary computational predictions of PMN activities confirm the likely necessity of strong ventral muscle coactivity for driving escape. Probing specific PMNs during rolling demonstrates robustness of circuits controlling escape and requires further investigation, alongside the role that sensory feedback could play in this behavior. Altogether, these data reveal a new circuit organization and motor activity pattern that underlie the coordination of muscles during an escape sequence. Future work could reveal circuit components necessary for escape, including the mechanistic basis for action selection, behavioral maintenance, and behavioral flexibility.