Summary

The walking and surface film locomotion of a terrestrial spider, Lycosa rabida Walckenaer, and a semi-aquatic spider, Dolomedes triton (Walckenaer), are compared. Stepping patterns during locomotion on a solid substrate resemble an alternating tetrapod in both species. L. rabida retains elements of the terrestrial gait when moving on water: propulsive thrusts are concentrated at the tarsi and intrasegmental legs step in alternation. However, protraction/retraction (p/r) values are generally greater and phase values of adjacent ipsilateral legs are lower compared to values for walking animals. D. triton shows greater behavioural specialization for aquatic locomotion: propulsive forces are generated along the length of the leg, intrasegmental legs move in synchrony and leg 4 (which is not used for propulsion on water) functions in yaw correction. Changes in p/r value and ipsilateral phase accompanying the transition from a solid substrate to the water surface are similar to those found in L. rabida. The fact that coordination within a segment and coordination between segments can be altered independently in both proximate and evolutionary time suggests that interleg coupling in spiders is governed by two distinct mechanisms: intrasegmental and intersegmental mechanisms. It is suggested that proximate changes in p/r value and ipsilateral phase in both spiders may be the effect of sensory modulation of similar motor control mechanisms and that certain specializations for aquatic locomotion in D. triton (such as yaw corrective movements of leg 4) are derived from elements of the primitive terrestrial motor programme.