Active tactile exploration and tactually induced turning in tethered walking stick insects.

Many animals use their tactile sense for active exploration and tactually guided behaviors such as near-range orientation. In insects, tactile sensing is often intimately linked to locomotion, resulting in the orchestration of several concurrent active movements, including turning of the entire body, rotation of the head, and searching or sampling movements of the antennae. The present study aims at linking the sequence of tactile contact events to associated changes of all three kinds of these active movements (body, head and antennae). To do so, we chose the Indian stick insect Carausius morosus, an organism commonly used to study sensory control of locomotion. Methodologically, we combined recordings of walking speed, heading, whole-body kinematics and antennal contact sequences during stationary, tethered walking and controlled presentation of an 'artificial twig' for tactile exploration. Our results show that object presentation episodes as brief as 5 s are sufficient to allow for a systematic investigation of tactually induced turning behavior in walking stick insects. Animals began antennating the artificial twig within 0.5 s, and altered the beating fields of both antennae in a position-dependent manner. This change was mainly carried by a systematic shift of the head-scape joint movement and accompanied by associated changes in contact likelihood, contact location and sampling direction of the antennae. The turning tendency of the insect also depended on stimulus position, whereas the active, rhythmic head rotation remained unaffected by stimulus presentation. We conclude that the azimuth of contact location is a key parameter of active tactile exploration and tactually induced turning in stick insects.