Changing 3D heading direction in terrestrial locomotion: Effectiveness of visual landmarks and the onset of turning in stick insects

Climbing animals need to be able to adjust their heading in both vertical and horizontal directions, requiring pitch and yaw rotation of the body axis, respectively. Stick insects are climbing herbivors that dwell among leaves and branches of the vegetation they feed on. By nature, they need to adjust their 3D heading in a complex environment. Since insects in general turn towards visual landmarks, we test whether or not stick insects initiate pitch rotation in response to landmarks that reliably elicit yaw rotation. We show that this is not the case. Instead, tactile cues alone are sufficient to mask the effect of visual deprivation, whereas lack of tactile cues strongly delays body inclination and the onset of climbing, even when same visual landmark cues are present that are sufficient to induce turning reliably. In a second set of experiments, we use a setup that constrains the onset of turning to tell whether visually induced yaw rotation is initiated by swing movements or rather by stance movements of the front legs. We show that a turn-related left-right asymmetry of front leg movement begins at least one stance phase after passing the constraint and may begin with the first swing movement thereafter. We conclude that stick insects do not exploit visual cues for initiating pitch rotation, despite they reliably initiate yaw rotation towards the exact same landmark. Moreover, we demonstrate that yaw rotation towards visual landmarks may begin with a swing movement of a front leg, much like the pitch rotation at the onset of climbing.