NONSPIKING LOCAL INTERNEURONS IN INSECT LEG MOTOR CONTROL .1. COMMON LAYOUT AND SPECIES-SPECIFIC RESPONSE PROPERTIES OF FEMUR-TIBIA JOINT CONTROL PATHWAYS IN SLICK INSECT AND LOCUST

1. Locusts (Locusta migratoria) and stick insects (Carausius morosus) exhibit different strategies for predator avoidance. Locusts rely prim arily on walking and jumping to evade predators, whereas stick insects become cataleptic, catalepsy forming a major component of the twig mi mesis exhibited by this species. The neuronal networks that control po stural leg movements in locusts and stick insects are tuned differentl y to their specific behavioral tasks. An important prerequisite for th e production of catalepsy in the stick insect is the marked velocity d ependency of the control network, which appears to be generated at the level of nonspiking local interneurons. We examined interneuronal pat hways in the network controlling the femur-tibia joint of the locust m iddle leg and compared its properties with those described for the sti ck insect middle leg. It was our aim to identify possible neural corre lates of the species-specific behavior with regard to postural leg mot or control. 2. We obtained evidence that the neuronal networks that co ntrol the femur-tibia joints in the two species consist of morphologic ally and physiologically similar-and thus probably homologous-interneu rons. Qualitatively, these interneurons receive the same input from th e femoral chordotonal organ receptors and they drive the same pools of leg motoneurons in both species. 3. Pathways that contribute to the c ontrol of the femur-tibia joint include interneurons that support both ''resisting'' and ''assisting'' responses with respect to the motoneu ron activity that is actually elicited during reflex movements. Signal processing via parallel, antagonistic pathways therefore appears to b e a common principle in insect leg motor control. 4. Differences betwe en the two insect species were found with regard to the processing of velocity information provided by the femoral chordotonal organ. Intern euronal pathways are sensitive to stimulus velocity in both species. H owever, in the locust there is no marked velocity dependency of the in terneuronal responses, whereas in the same interneurons of the stick i nsect it is pronounced. This characteristic was maintained at the leve l of the motoneurons controlling the femur-tibia joint. Pathways for p ostural leg motor control in the locust thus lack an important prerequ isite for the generation of catalepsy, that is, a marked velocity depe ndency.