Fig. 1. (A) Overlay of all tracked frames from a single walking sequence. Tracked dots mark the distal ends of the femora and the tibiae of all legs, the head is pointing to the right. The animal was performing a left turn in the depicted sequence. (B) Schematic drawing of the stick insect with the points tracked. x-values always denote points along the axis of the animal whereas y-values mark points perpendicular to the animal. The x₀-value was always set at the level of the coxae for each leg to give a clear reference point. As an example for the determination of the vector for step length and direction, the right middle leg is drawn in two arbitrary positions, one anterior extreme position (ML–AEP) and one posterior extreme position (ML–PEP). The vectors for all steps connecting the two positions, normalized to the origin in the AEP, gave direction in deg. and step length in mm. The 0–180 deg. axis was always parallel to the body axis and crossed the AEP, 270 deg. always points away perpendicularly. ML, middle leg; FL, front leg; HL, hind leg.