Segment-specific and state-dependent targeting accuracy of the stick insect

In its natural habitat, Carausius morosus climbs on the branches of bushes and trees. Previous work suggested that stick insects perform targeting movements with their hindlegs to find support more easily. It has been assumed that the animals use position information from the anterior legs to control the touchdown position of the ipsilateral posterior legs. Here we addressed the question of whether not only the hindleg but also the middle leg performs targeting, and whether targeting is still present in a walking animal when influences of mechanical coupling through the ground are removed. If this were the case, it would emphasize the role of underlying neuronal mechanisms. We studied whether targeting occurred in both legs, when the rostral neighboring leg, i.e. either the middle or the front leg, was placed at defined positions relative to the body, and analyzed targeting precision for dependency on the targeted position. Under these conditions, the touchdown positions of the hindlegs show correlation to the position of the middle leg parallel and perpendicular to the body axis, while only weak correlation exists between the middle and front legs, and only in parallel to the body axis. In continuously walking tethered animals, targeting accuracy of the hindlegs and middle legs parallel to the body axis barely differed. However, targeting became significantly more accurate perpendicular to the body axis. Our results suggest that a neural mechanism exists for controlling the touchdown position of the posterior leg but that the strength of this mechanism is segment specific and dependent on the behavioral context in which it is used