The control of body position in the stick insect (Carausius morosus), when walking over uneven surfaces

Abstract

Abstract. An investigation has been made of the way, in which the height of the body of an insect (Carausius morosus) is controlled when walking over an uneven terrain. The animals have been filmed from the side while walking over different types of irregularity (step up, step down, obstacle, ditch). A frame by frame analysis of the height of the three thoracic segments of the insect has been performed. A computer model has been set up, which is able to describe the experimental results within the exactness of measurement. This model consists of three independent height controllers, each having a unique characteristic. The coupling of these three controllers is performed mechanically. One possible interpretation of this model is that the height of each segment is controlled by a closed loop mechanism with a proportional element as a controller. A. IntroductionUntil now there exist very few experimental results dealing with the control of body position in free walking insects. Recent investigations describe only menotactic control of the longitudinal axis of the body relative to directed stimuli, e.g. gravity or light (B~issler, 1975; see here for further references, Wendler, 1972). The present paper describes the manner in which the body position is influenced by the shape of the walking surface. This problem has only been investigated up to now in an indirect way. For example B~issler (1965, 1967, 1972a, b, 1973, 1974) and Wendler (1964Wendler ( , 1972 have examined the mechanisms, that control the angle of leg joints in standing or fixed animals. Although it may be of great practical interest for the construction of walking machines able to travel over an irregular terrain, one has no idea about the mechanisms which * Supported by the Deutsche Forschungsgemeinschaft a six-legged animal applies when walking over such a surface. Only Okhotsimskiy et al. (1972) have constructed a mathematical model describing a six-legged apparatus able to run over uneven terrain, by using data which have been obtained by measuring the coordination of the legs of an insect (Wilson, 1966). The decisive disadvantage of Okhotsimskiy's model is, that it only works, if the shape of the walking surface does not deviate from a given horizontal line by more than in a defined amount. This depends on the length of the leg-limbs. As moreover the data of Wilson are only concerned with the coordination of the six legs, the present investigation investigates the way, in which a free walking insect changes its body position when walking over several differently shaped obstacles. From the preliminary results a model is formulated to describe the body-movements of the animal under the investigated conditions. B. MethodThe animals used in this investigation were adult stick insects (Carausius morosus, Phasmids) reared at this institute. In order to be able to evaluate the height of the different parts of the body of the walking insect, the animals have been filmed with a super 8 camer~ (18 frames/s) from the side, when walking over a horizonta...

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