Locomotory behavior in the hawkmoth Manduca sexta: kinematic and electromyographic analyses of the thoracic legs in larvae and adults

During metamorphosis in Manduca sexta, muscles and most sensory structures of the thoracic legs undergo extensive changes while the motor neurons that are present in the larva persist into the adult. The main goal of this work was to identify similarities and dissimilarities in thoracic leg movements during crawling in larvae and walking in adults. This information provides a foundation for understanding the extent to which centrally located neural elements are reorganized during metamorphosis to accommodate changes in locomotion. Analysis of electromyographic activity from leg muscles synchronized with video-taped recordings of the leg movements during larval crawling and adult walking revealed differences in cycle periods as well as intersegmental and intrasegmental patterns of coordination. Larval crawling was characterized by synchronous movements of segmental pairs of legs as activity proceeded slowly from the move posterior to the more anterior segments. During crawling, antagonistic muscles maintained a strict reciprocity. In contrast, walking in adults was characterized by fast, alternating movements of the left and right prothoracic legs and more variable coordination patterns in the mesothoracic and metathoracic legs (ranging from synchrony to alternation). In adults, sensory information, possibly associated with the weight-bearing or postural demands of walking on an incline, contributed to a strong dependence between the duration of muscle activity and cycle period and to the extent that the muscle activity overlapped during walking.