Leg co-ordination during walking in the crab,Uca pugnax

Summary

• 1. Cine films of fiddler crabs (Uca pugnax) walking sideways on land were analysed frame by frame in order to study the co-ordination of the legs both during normal walking and following amputation (induced autotomy) of one of the 3rd pair of legs.

• 2. Although the average gait of unoperated animals approximates to an alternating tetrapod rhythm, equivalent to the alternating tripod gait of insects (Figs. 3–7), the stepping order of legs on leading and trailing sides of the animal is different. The dominant sequence on the leading side is 2435 (legs numbered from front to rear), that on the trailing side 2534 (Table 1), the chelae (leg 1) being usually held off the ground.

• 3. Stepping parameters also change with walking speed (Figs. 8, 10), the most clear-cut of these changes being the reduced relative duration of the power stroke at high walking speeds. At low walking speeds, clear-cut metachronal co-ordination is sometimes seen in trailing legs (Fig. 9), but the data nevertheless do not fit Wilson's 1966 metachronal model for insect locomotion.

• 4. Amputation of one of the 3rd pair of legs causes adaptive changes in the gait. The most obvious of these changes are that the chelae are now frequently used in walking and legs 4 and 2 on the operated side of the animal now alternate whereas before they moved together (Figs. 11–13, Table 2). Evidence is presented to show that these changes are brought about by the changed sensory input and are not due either to the effects of axon section on motorneurone electrical excitability, or to the animal walking more slowly and using a different but normally occurring gait.