Fruit flies alter flight direction by generating rapid, stereotyped turns, called saccades. The successful implementation of these quick turns requires a well-tuned orchestration of neural circuits, musculo-skeletal mechanics, and aerodynamic forces. The changes in wing motion required to accomplish a saccade are quite subtle, as dictated by the inertial dynamics of the fly's body. A fly first generates torque to begin accelerating in the intended direction, but then must quickly create counter-torque to decelerate. Several lines of evidence suggest that the initial turn is initiated by visual expansion, whereas the subsequent counter-turn is triggered by the gyroscopic halteres. This integrated analysis indicates how the functional organization of neural circuits controlling behavior is rigidly constrained by the physical interaction between an animal and the external world.