PUB - Publikationen an der Universität Bielefeld

Insects are capable of efficient locomotion in a spatially complex environment, such as walking on a forest floor or climbing in a bush. One behavioural mechanism underlying such adaptability are searching-movements after loss of ground contact. Here, the kinematic sequence of leg searching-movements of the stick insect Carausius morosus is analysed. Searching-movements are shown to be stereotypic rhythmic movement sequences consisting of several loops. The typical loop structure allows calculation of a mean tarsus trajectory by use of a feature-based averaging procedure. Thus, description of the common underlying structure of this movement pattern is possible. Phase-relationships between joint angles, analysed for searching front legs, indicate a central role of the thorax-coxa-joint in searching-movements. Accordingly, the stereotyped loop structure of searching differs between leg types, with leg-specific patterns being caused by differing protraction-retraction movements in the thorax-coxa-joint. A simple artificial neural network that originally had been devised to generate simple swing movements, allows to simulate two essential features of empirical searching-trajectories: (1) cyclic movements and (2) smooth transition into a search as a non-terminated swing-movement. Whereas it is possible to generate several loops of a middle leg search, the exact size and shape of the loops falls short of a real-life approximation. Incorporation of front leg retraction or hind leg protraction during searching will also require an extension to the current model. Finally, front leg searching is backed up by antennal movements. Also, because leg searching-movements are a local behaviour, the remaining legs on ground continue to stance, causing a forward shift of the body, including the searching leg. Due to this shift, the centre of the searched space is close to the anterior extreme position of the tarsus during walking, representing the location of most likely ground contact according to past experience. Therefore, the behavioural relevance of searching-movements is a matter of combined action of several limbs.