Visual, tactile and proprioceptive cues for spatial coordination of the legs in insect locomotion.

Research Areas: 

Stick insects spatially coordinate their six legs when climbing around in their complex environment, the canopy. To solve this task, the nocturnal animals make use of different sensory modalities, in particular proprioception, touch and vision. The integration of specific information coming from the eyes, the antennae and various proprioceptors and its role for climbing is little understood, although it is clear that all of these sensory inputs are used in locomotion. Here, we study the role of active movements of head, antennae and legs, and the relative influence of three sensory modalities in spatial leg coordination.


Methods and Research Questions: 

(1) Do middle and hind legs use proprioceptive information from the anterior leg to adjust their touch-down position? (2) Do active head movements couple visual and tactile senses or rather affect one sense more than the other? (3) What is the relative importance of vision, touch and proprioception in leg coordination during climbing?

The stick insect Carausius morosus lives in the canopy of trees and bushes, where sensory information is crucial for save and stable locomotion. Whereas it is clear that tactual information from the antennae is used for spatially coordinated leg movements, the relative contribution of visual and proprioceptive cues, and the integration of multi-modal information are far less understood. Apart from kinematic analyses of leg movements in setups with and without visual landmarks, we are particularly interested in the role of the head in coupling/de-coupling of vision and touch.. Since the head caries the easy and the antennae, head movements affect the effective working ranges of both senses, and may stabilize or de-stabilize gaze and antennal beating field against body movements. To date, the role of active head movements and vision in spatial leg coordination is unknown. The antennal movement pattern seems to play a major role for spatial coordination of the front legs. However, middle and hind legs cannot reach the contact points of the antennae and, therefore, depend on other sources of spatial information. Potential candidates for this are sensory hair fields and other proprioceptors in the joints of adjacent legs. The overall goal of the study is to understand the relative contribution of three sensory modalities in spatial coordinate transfer to and among the six legs.

Experiments are carried out with unrestrained walking and climbing stick insects of the species Carausius morosus, using different setups (e.g., steps of different height and gaps of different width). A Vicon motion capture system (Vicon, Oxford, UK) is used to track retro-reflective markers, attached to the unrestrained moving stick insects. The three-dimensional marker coordinates, the position with respect to the segment to which they are attached, and the segment lengths of all body parts are used for a kinematic reconstruction of the joint angle time courses in thorax, neck, antennae and legs. The motion capture recordings with a temporal frequency of 200 Hz and a maximum spatial acuity of 0.1 mm yield to precise information about the whole-body movements. The 3D reconstruction of the stick insect’s compound eye will allow the calculation of the perceived size and location of visual landmarks on the setup. Markers on each antenna track the antennal motion pattern, allowing the reconstruction of the antennal contact pattern during tactile sampling.



The first result is that the touch-down positions of hind and middle legs are correlated in all three dimensions with the foot positions of the ipsilateral middle and front legs, respectively. This shows that the “targeting mechanism” that was first postulated by Cruse (1979) also holds for three-dimensional climbing situations. Previously, it has been shown that planar “targeting” of hind and middle legs is dependent on proprioceptive information from hair fields in the leg joints. Furthermore, it is known that the antennal movement patterns change after the detection of an object, indicating that the antennae are used to scan the object for appropriate foot contact positions. Indeed, there is evidence that front legs adjust their touch-down positions according to antennal contacts (Schütz and Dürr, 2011). Occlusion of the eyes does not abolish this kind of spatial adjustment, however, it is still unclear whether visual cues can improve performance.