SUMMARY Aquatic nymphs of the mayfly Centroptilum triangulifer produce ventilatory flow using a serial array of seven abdominal gill pairs that operates across a Reynolds numbers (Re) range from 2 to 22 during ontogeny. Net flow in small animals is directed ventrally and essentially parallel to the stroke plane (i.e. rowing), but net flow in large animals is directed dorsally and essentially transverse to the stroke plane (i.e. flapping). Detailed flow measurements based on Particle Image Velocimetry (PIV) ensemble-correlation analysis revealed that the phasing of the gills produces a time-dependent array of vortices associated with a net ventilatory current, a fluid kinematic pattern, here termed a ‘phased vortex pump’. Absolute size of vortices does not change with increasing animal size or Re , and thus the vortex radius ( R v ) decreases relative to inter-gill distance ( L is ) during mayfly growth. Given that effective flapping in appendage-array animals requires organized flow between adjacent appendages, we hypothesize that rowing should be favored when L is / R v <1 and flapping should be favored when L is / R v >1. Significantly, the rowing-to-flapping transition in Centroptilum occurs at Re ∼5, when the mean dynamic inter-gill distance equals the vortex radius. This result suggests that the Re -based rowing–flapping demarcation observed in appendage-array aquatic organisms may be determined by the relative size of the propulsive mechanism and its self-generated vortices.

SUMMARY Certain leg joints in arachnids lack extensor muscles and have elastically deformable transarticular sclerites spanning their arthrodial membranes, an arrangement consistent with a model in which flexor muscles load transarticular sclerites during flexion and energy from elastic recoil is used for extension. This study quantifies the potential contribution of elastic recoil to extension torque at joints of the fourth leg of representative arachnids. Extension torques of isolated joints with and without transarticular sclerites were measured as the joint was rotated through angles and at angular velocities comparable with those used by walking animals. The procedure was repeated with the joint subjected to different internal fluid pressures in order to assess the potential role of hydraulically induced extension. The efficiency of elastic energy storage (resilience) in the absence of internal fluid pressure was 70-90% for joints with well-developed transarticular sclerites, and the magnitude of torque was similar to those produced by different joint extension mechanisms in other arthropods. Increased internal fluid pressure acted synergistically with transarticular sclerites in some joints but had little or no effect in others. Joints that lacked both extensor muscles and transarticular sclerites appeared to be specialized for hydraulic extension, and joints operated by antagonistic muscles lacked apparent specializations for either elastic or hydraulic extension. It is well known that elastic energy storage is a significant contributor to propulsion in running vertebrates and certain arthropods, where elastic elements are loaded as the center of mass falls during one phase of the locomotor cycle. However, transarticular sclerites are apparently loaded by contraction of flexor muscles when the leg is not in contact with the substratum. Hence the mechanism of a transarticular sclerite is more similar to the flight and jumping mechanisms of other arthropods than to running vertebrates. The evolutionary significance and potential mechanical advantages of the transarticular elastic mechanism are discussed.

Activity patterns of six morphologically similar, homologous muscles were recorded from the fourth leg pair in two species of arachnids that use different mechanisms to extend the femur-patella (knee) joint during locomotion. The giant whipscorpion Mastigoproctus giganteus (Uropygi) lacks femur-patella extensor muscles but extends this joint with hydraulic pressure, a mechanism that appears to be phylogenetically primitive in arachnids. The black emperor scorpion Pandinus imperator (Scorpiones) has an unusual muscle that promotes simultaneous extension at the femur-patella joint and distally adjacent patella-tibia joint. Comparison of electromyograms from freely walking animals revealed similarities in muscle firing patterns of the two species, including asymmetrical alternation of trochanter-femur levator and depressor muscles, transfemoral muscle activation during protraction and abrupt termination just prior to levator activation, and persistence of flexor activity during retraction (extension). These results indicate that the apparent evolutionary transition from hydraulic to muscular extension occurred without substantial alteration of several components of the primitive motor programme. It is suggested that the patterns of extension force generated by hydraulic pressure and by bifunctional extensors are similar and that this similarity is associated with conservation of the motorprogramme.

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.