Turning and righting inGeotrupes (Coleoptera, Scarabaeidae)

Abstract

1. Probing movements of righting occur when most of the legs lose contact with the ground in the propulsive phase of step. The essential feature of the probing movements is the raising of the legs to the extreme dorsal position (Figs. 3, 6). The coordnation between the joints of a leg is different from the coordination during walking (Table 2). Sometimes the forelegs continue the walking movements during righting. 2. During the probing movements the beetle keeps the alternating tripod gait, the coordination is less rigid now than during walking. The forelegs exhibit tendency to double steps (Fig. 5). 3. The walking beetles respond to the rotation of a striped pattern with optomotor turning (Fig. 1). Under a strong visual stimulation the middle leg on the inner side of the turning arc begins to pull laterally while the front and hind legs walk backwards. The head turns according to the moving pattern. 4. In the process of righting 90% of beetles show no optomotor response (Figs. 8, 9). In several beetles this response was observed in the first thoracic segment (turning of the head, asymmetric leg movements); the middle and hind legs continued the righting movements (Figs. 10, 11). 5. Directionally-sensitive neurons in the optic lobe measure the motion of a striped pattern with equal gain during different locomotory states: forward walking, at rest, during righting, and during active turning (Fig. 12). 6. The results are discussed in relation to the stimuli which elicit the stereotype of righting, the role of central automatism and reflexes which control the walk and the righting, and the level of interaction of righting and turning commands.