Load-compensating reactions in the proximal leg joints of stick insects during standing and walking

The responses of retractor coxae and protractor coxae motoneurones and of the retractor coxae muscle to cuticular stress applied to the leg were investigated in standing and walking stick insects, Carausius morosus. The coxa of a middle or hind leg was restrained and the trochanterofemur was bent by moving the distal tip of the femur anteriorly or posteriorly, i.e. perpendicular to its normal plane of movement. The maximum amplitude used was 200mm, which corresponds to a deflection of 0.95˚, and the forces necessary to bend the trochanterofemur were between 0.29 and 2.91mN. Thus, cuticular stress could be applied in particular directions and with controlled amplitudes within the physiological range. This cuticular stress induced direction- and amplitude-dependent reflex responses in excitatory retractor coxae and protractor coxae motoneurones. The reflexes clearly constitute a negative feedback system which continuously compensates cuticular stress in the legs of standing and walking animals. Two groups of trochanteral campaniform sensilla, the posterior group and the anterior ventral group, were shown to underlie this feedback loop. These results prove directly for the first time the important function of single groups of trochanteral campaniform sensilla in the control of posture and locomotion in stick insects. I discuss the importance of these results for the interpretation of previous findings on stick insects subjected to increased load during walking