Regulation of the body-substrate-distance in the stick insect: Step responses and modelling the control system

The feed back mechanism subserving the regulation of the body-substrate-distance in the stick insect Carausius morosus has been investigated by means of step-like stimuli. Based on the results obtained in open-loop experiments a model is developed which describes the results obtained under closed loop conditions. When the experimental animal is pushed or pulled in dorso-ventral direction an initial fast and a subsequent very slow change of the body height z over the substrate are observed. The late slow response is a nearly linear function of time and can last for more than one hour if the animal is pulled with moderate forces. It withstands less effectively if pushed. During the slow phase of the response sudden changes of z and of the slope of the z(t)-curves occur, presumably due to corresponding changes of the amplification within the feed back loop.