Combined kinematic and electromyographic analyses of proleg function during crawling by the caterpillar Manduca sexta

The planta retractor muscles in the prolegs of Manduca sexta caterpillars a re a frequently-used model system for investigating a number of problems in neurobiology. We have combined kinematic and electromyogram analysis of pr oleg movements during crawling to examine the roles of these muscles during normal behavior. We found that retractor muscle activity is highly stereot yped, and that the primary function of these muscles is to disengage the cr ochets at the tip of the proleg for the swing phase of crawling. The durati on of activity of the muscles was tightly coupled to the phasing of crawlin g behavior. The stepping patterns of animals changed to accommodate variati ons in the substrate, but the relative timing of retractor muscle activity was unaffected. There were no clear correlations between the various proper ties of motoneuronal input to the muscle (duration of activity, number of s pikes, peak frequency of spikes) and the resulting muscle length change. Pe rhaps because it functions partially as a hydrostat, this may represent a n euromuscular system in which a significant part of the control algorithm is embedded in its morphology.