Fig. 1. Optomotor-induced curve walking behaviour. (A) Stick insects walked on the apex of a light-weight sphere that floated on an air cushion. Animals were tethered to a support that required them to carry their own weight by adjustment of body height, but restricted horizontal translation and rotation. A vertical stripe pattern was rotated around the animal, reliably eliciting an optomotor turning response in the direction of stimulus motion. (B) Walking legs are denoted as outer legs (O1 to O3) and inner legs (I1 to I3), depending on turning direction (broken arrow). For example, in clockwise rotations left legs are outer legs and right legs are inner legs. (Ci–Cviii) Examples of walking paths (left) and corresponding gait patterns (right) of each one of the eight animals used in this study. Walking paths: Head position (circles) and orientation of the body axis (line segments) are indicated for every 20th record of the tracking system. Open and filled symbols indicate the pre-stimulus period (first 7.5 s) and stimulus period (last 15 s), respectively. Width of the scale grid, 10 cm. Path curvature is always such that a loop in the path remains within one field of the grid. Gait patterns: Stance phases (power strokes) of each leg are indicated by a row of black bars. Rows from top to bottom show left front, middle and hind leg, followed by right front, middle and hind legs, respectively. Time runs from left to right, with vertical lines spaced by 7.5 s, indicating the pre-stimulus, transition and curve-walk period of each trial. The bold vertical line marks stimulus onset. Gait patterns are sometimes time-varying within single stimulus periods and vary strongly between animals, even if the walked path is fairly similar (e.g. compare Ci, Cvi and Cvii). Red diagonal lines highlight back-to-front waves of step cycles during tetrapod-like gait, blue vertical lines highlight in-phase step cycles of ipsilateral front and hind legs during tripod-like gait.