Leg Coordination during Walking in Insects

Locomotion depends on constant adaptation to different requirements of the environment. An appropriate temporal and spatial coordination of multiple body parts is necessary to achieve a stable and adapted behavior. Until now it is unclear how the neuronal structures can achieve these meaningful adaptations. The exact role of the nervous system, muscles and mechanical constrains are not known. By using preparations in which special forms of adaptations are considered under experimental conditions that selectively exclude external influences, like mechanical interactions through the ground or differences in body mass, one can draw conclusions about the organization of the respective underlying neuronal structures. In the present thesis, four different publications are presented, giving evidence of mechanisms of temporal or spatial coordination of leg movements in the stick insect Carausius morosus and the fruit fly Drosophila melanogaster during different experimental paradigms. At first, state dependent local coordinating mechanisms are analyzed. Electromyographic measurements of the three major antagonistic leg muscle pairs of the forward and backward walking stick insect are evaluated. It becomes evident that only the motor activity of the most proximal leg joint is changed when walking direction is changed from forward to backward, which demonstrates that the neuronal networks driving movement in each individual leg seem to be organized in a modular structure. In the second part mechanisms that influence movement speed of the individual leg and coordination of speed between the different legs of the stick insect come into focus. Electrophysiological and behavioral experiments with the intact and reduced stick insect were used to examine relationships between the velocity of a stepping front leg and neuronal activity in the mesothoracic segment as well as correlations between the stepping velocities of different legs during walks with constant velocity or with distinct accelerations. It was shown that stepping velocity of single legs were not reflected in motoneuron activity or stepping velocity of another leg. Only when an increase in walking speed was induced, clear correlation in the stepping velocities of the individual legs was found. Subsequently, the analysis of changes in temporal leg coordination during different walking speeds in the fruit fly reveals that the locomotor system of Drosophila can cover a broad range of walking speeds and seems to follow the same rules as the locomotor system of the stick insect. Walking speed is increased by modifying stance duration, whereas swing duration and step amplitude remain largely unchanged. Changes in inter-leg coordination are gradually and systematically with walking speed and can adapt to major biomechanical changes in its walking apparatus. In the final part it was the aim to understand the role of neuronal mechanisms for the orientation and spatial coordination of foot placement in the stick insect. Placement of middle and hind legs with respect to the position of their respective rostrally neighboring leg were analyzed under two different conditions. Segment and state dependent differences in the aiming accuracy of the middle and hind legs could be shown, which indicate differences in the underlying neuronal structures in the different segments and the importance of movement in the target leg for the processing of the position information. Taken together, common principles in inter-leg coordination where found, like similarities between different organisms and segment specific or state dependent modifications in the walking system. They can be interpreted as evidence for a highly adaptive and modular design of the underlying neuronal structures.