Locomotion in a forward walking crab

Summary

• 1. Mictyris platycheles, the soldier crab, walks predominantly forward (Fig. 2). Two-thirds of the crabs studied used all 10 legs in walking; the remainder held up the chelae and walked with 8 legs.

• 2. The soldier crab is egg-shaped in contrast to laterally elongated sideways-walking crabs. Its joints which allow anterior-posterior leg movement describe larger angles than in sideways-walking crabs of a similar size, while its joints which permit lateral movement describe smaller angles (Fig. 1).

• 3. Mictyris walks mainly with two gaits: one overlapping (2534 or equivalents; >50% of the time) and the other metachronal (2345 or equivalents; >30% of the time) (Tables 1 and 2). It uses both at all but the highest stepping frequencies at which the overlapping gait predominates (Fig. 5).

• 4. Crabs stepped between 2 and 4 steps/s. With increasing rate the durations of both power and return strokes, which are nearly equal in each leg except leg 1, decreased (Fig. 7).

• 5. Ipsilateral legs 1 and 4 in 10-legged crabs and ipsilateral legs 2 and 5 in all crabs tended to step together (Fig. 8). Adjacent legs on the same side of the body stepped neither in phase nor in antiphase but with a mean phase relationship of 0.33. Contralateral legs of the same segment stepped nearly in opposite phase (Fig. 6 and Table 3).

• 6. The stepping pattern (Figs. 3, 4 and 9) is generated by alternation at one-sixth of a step cycle between sets of one or two legs on opposite sides of the body. At any given moment during walking crabs support themselves by various numbers of legs.

• 7. To walk forwardMictyris uses strategies of promotion and remotion similar to those employed by long-bodied decapod crustaceans. It also uses the same gaits and phase relationships on both sides of the body. In these respects the walk ofMictyris more nearly resembles those of other forward-walking animals rather than sideways-walking crabs.