Leg coordination in the stick insect Carausius morosus: effects of cutting thoracic connectives

Behavioral studies of stick insects have identified six mechanisms which coordinate leg stepping. All six are active between ipsilateral leg pairs. As a first step towards locating the neurons mediating these interactions, the present study describes the effects of cutting one of the paired thoracic connectives. After the operation the following changes in step coordination occurred. The ipsilateral leg immediately caudal to the severed connective generally showed weak stepping. In free-walking animals it often remained near its posterior extreme position and dragged along the substratum. During supported walking, rhythmic stepping was common, but the swing phase of this leg was longer and both temporal and spatial coordination were disturbed. When the leg made a pause it usually stopped in the air near the end of its swing movement. During steady walking, the operation interrupted information from the adjacent forward leg normally used to guide the end-point of the swing or to signal errors in leg placement and elicit a correctivetreading-on-tarsus reflex. It also interrupted position information affecting the start of the swing. For the leg rostral to the cut, the inhibition during the swing of the posterior leg and the excitation when the latter started its retraction were both interrupted. These results indicate that all six ipsilateral coordination mechanisms are primarily mediated by the ipsilateral connective. In addition, the data show that contralateral coordination within the segmental ganglion is strongest for the front legs, weaker for the rear legs, and not discernible for the middle legs.

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