Insects Are Both Impeded and Propelled by Their Legs during Walking

It has been known for several years (Graham, 1972) that some insects do not walk with a constant body velocity. Rapid variations in forward velocity during each leg cycle have been found in locust (Burns, 1973), grasshopper (D. Graham, unpublished) and cricket (Weber, Thorson & Huber, 1981). The velocity variations are complex at slow speeds but in the tripod step pattern, used at the highest speeds, two clear maxima and minima appear in each leg cycle. In the stick insect the velocity minimum is usually close to zero and the maximum velocity of the body occurs near the middle of each retraction stroke. The behaviour is markedly different from that reported for walking in man, monkey and the dog where the maximum speed of the body is achieved at the end of the retraction stroke (Alexander, 1976; Cavagna, Heglund & Taylor, 1977) and the momentum of the body is maintained while recovering the supporting legs. The present paper examines the velocity developed by the legs on each side of the body when walking on a pair of independent treadwheels and compares these results with the force platform measurements of Cruse (1976) and the motor output to the middle leg during walking on a self-propelled tread-wheel (Graham & Wendler, 1981). The results indicate that walking locomotion in the stick insect, and probably in many other insects, has special characteristics quite different from those of vertebrate locomotion, resulting in a rather bizarre walking behaviour which could be described as 'lurching' locomotion. It appears to be essential that the animal achieve a static support condition each time the recovered legs touch the walking surface. This 'braking' of forward movement is clearly demonstrated in slow motion films of freewalking animals. A physiological correlate of this effect is observed under conditions where the apparent inertia of the body can be increased. For example, walks on heavy