Extended Data Fig. 3: Adhesion and contact forces during walking on hilly terrain.

a, Schematic diagram of leg-floor contact forces for the fly model standing on an inclined surface. The adhesion actuator injects force in the normal direction, which in response increases the normal component of the contact force. This creates a larger margin between the tangential contact force component which resists slipping and the slip threshold (the friction cone boundary). b, Time-lapse of the trained policy rollout of the RL task where the fly model learns to use the adhesion mechanism to overcome sine-like hills. All the following panels correspond to this policy rollout. c, Adhesion actuator forces generated by the fly’s claws during the policy rollout shown in b. In our model, the largest adhesion force per leg is one fly body weight. The fly body weight, mg = 0.96 dyn, is shown for comparison. The leg-floor contacts are shown for clarity. d, Contact force norm during the policy rollout. The fly weight is shown for comparison. e, The difference between the slip threshold force and the tangential component of the contact force. This is the “margin” available to resist slipping under external forces and propulsion generation. f, The difference between the actual tangential contact force and the largest tangential contact force that would have been available without adhesion. Positive means the contact would have slipped without adhesion. Negative means the contact would not have slipped without adhesion (that is, the contact is inside the friction cone already without adhesion).