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Fig. 1. Schematic drawings of the experimental setup and behavioural contexts. (A)
Right front leg of a stick insect with thorax-coxa joint (ThC),
coxa-trochanter joint (CTr) and femur-tibia joint (FTi) indicated. Each joint
is responsible for different motion components of the leg: ThC,
protraction/retraction (pro/re); CTr, levation/depression (lev/dep); FTi,
extension/flexion (ex/fl). During swing movements (curved dotted arrow), the
leg moves through the air from the posterior extreme position (PEP) to the
anterior extreme position (AEP). During stance movements (straight broken
arrow), the leg pushes the body forward, moving from AEP to PEP in body
coordinates. An obstacle (grey probe) was held into the swing trajectory to
examine effects of perturbation. (B) Schematic trajectories of typical swing
movements of the right front leg (arrows) and approximate areas of PEPs and
AEPs. In straight walking (black trajectory), front leg movements are
symmetrical and approximately parallel to the body long axis, whereas in curve
walking, leg movements differ between inner (orange trajectory) and outer
(purple trajectory) front legs. The coordinate system used for analyses has
its origin between the front leg coxae. Curve walking was elicited by means of
a large-field visual motion stimulus of a rotating stripe pattern. It was
always the right front leg that was perturbed.
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