The function of the legs in the free walking stick insect,Carausius morosus

Summary

• 1. The function of the legs of a free walking mature stick insect (Carausius morosus) is investigated in four different walking situations: walks on a horizontal path, walks on a horizontal plane, walks on a horizontal beam with the body hanging from the beam and walks up a vertical path.

• 2. The geometrical data, which are necessary to describe the movement of the legs, are determined (Tables 1, 2, 3, 4; Figs. 2, 3, 4, 5).

• 3. The forces, by which the leg of a free walking animal acts on the walking surface, are measured (Table 5). Typical results are shown in Figures 6, 7, 8, 9 for each walking situation. From these forces and the known geometrical relationships the torques, which are produced by the antagonistic muscle systems at each leg joint, can be calculated (Fig. 10). Those torques calculated for different typical leg positions are shown in Table 6, 7, 8, 9 for each walking situation.

• 4. The results show that many things change depending upon the particular walking situation: the angular range in which the leg is moved (Table 2, Fig. 4), the activation and the kind of predominance of the antagonistic muscles (Table 6, 7, 8, 9), and especially the function of the single legs. Additionally, when looking at the direction of movement of a limb one cannot say which of the antagonistic muscles is predominating. Sometimes just the muscle opposite to the actual movement predominates (Table 7).

• 5. For two walking situations the function of the legs can be demonstrated in a simple way. In a walk on the horizontal plane: the forelegs mainly have feeler function, the middlelegs have only supporting function, while the hindlegs have supporting as well as propulsive function. In a walk with the body hanging from the horizontal beam: forelegs and hindlegs are used mainly to support the body, while the middlelegs additionally provide the propulsive forces.

• 6. In walking up the vertical path all legs provide support and propulsive forces. When walking on the horizontal path fore- and middlelegs on the

one hand and hindlegs on the other form the static construction of a three centered arch (Fig. 11). In the same way when the insect walks hanging from the horizontal beam, a hanging three centered arch is assumed. The importance of this construction is discussed.