Abstract
The paths followed by grain weevils (Sitophilus granarius) in a featureless environment are curved or circular. Animals walking in circles display average angular velocities that may exceed 10°/s. The circling is not due to any asymmetrical damage to the weevil's limbs. Circling weevils walking on a sloping surface follow characteristic paths: on slight slopes the circles become cycloids, which in turn, become relatively straight paths on steep slopes. The mean direction of the cycloidal paths deviates from the direction of the straight paths. Left circlers deviate to the left and right circlers deviate to the right. These results can be explained by a feedback control loop with an additive interaction of internal and feedback signals. The cycloid paths thus result from an unstable state of the feedback loop in which the feedback signal from the gravity receptors is always smaller than the internal signal.
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