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

Abstract

SUMMARYIn 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. Supplementary material available online at

Spatial coordination of foot contacts in unrestrained climbing insects

“…2A: see overlap of touch-down and target areas). Our findings confirm results on tethered walking animals (Dean and Wendler, 1983;Wosnitza et al, 2013). In contrast to these earlier studies, our animals had to support their own body weight and could move freely.…”

“…In part, this might be linked to increased variability in body orientation throughout single steps. Interestingly, precision of middle leg targeting improved during climbing, potentially hinting at a similar state dependency as was described for the onset of locomotion (Wosnitza et al, 2013).…”

“…Later, targeting was shown for walking insects, too, both for hind-and middle legs (Dean and Wendler, 1983). Recently, state dependency was reported, based on the comparison of standing and stationary walking (Wosnitza et al, 2013). In comparison, we show that spatial co-ordination among legs persists in freely walking and climbing animals, even if body orientation varies considerably and each leg's contribution to propulsion and body support changes.…”

“…The underlying 'targeting mechanism' makes the hindleg touch down close to the current position of the ipsilateral middle leg (Cruse, 1979). The same mechanism, though with lower precision, was found for the middle legs, which follow the position of the front legs (Dean and Wendler, 1983;Wosnitza et al, 2013). During visually induced curve walking, the targeting mechanism is affected differently, depending on whether it is an inner or an outer leg with respect to the curvature (Ebeling and Dürr, 2006).…”

Six-legged walking in insects: how CPGs, peripheral feedback, and descending signals generate coordinated and adaptive motor rhythms

“…Rule 4. This rule is responsible for spatial coordination and is active in posterior direction between ipsilateral legs (Cruse 1979;Dean and Wendler 1983;Wosnitza et al 2013b). A leg's current tarsal touchdown position influences the prospective touchdown position of a directly posterior leg; this rule acts as a targeting mechanism between a middle leg and a hind leg, and a front leg and a middle leg.…”