SUMMARY Argyroneta aquatica is a unique air-breathing spider that lives virtually its entire life under freshwater. It creates a dome-shaped web between aquatic plants and fills the diving bell with air carried from the surface. The bell can take up dissolved O 2 from the water, acting as a ‘physical gill’. By measuring bell volume and O 2 partial pressure ( P O 2 ) with tiny O 2 -sensitive optodes, this study showed that the spiders produce physical gills capable of satisfying at least their resting requirements for O 2 under the most extreme conditions of warm stagnant water. Larger spiders produced larger bells of higher O 2 conductance ( G O 2 ). G O 2 depended on surface area only; effective boundary layer thickness was constant. Bells, with and without spiders, were used as respirometers by measuring G O 2 and the rate of change in P O 2 . Metabolic rates were also measured with flow-through respirometry. The water–air P O 2 difference was generally less than 10 kPa, and spiders voluntarily tolerated low internal P O 2 approximately 1–4 kPa before renewal with air from the surface. The low P O 2 in the bell enhanced N 2 loss from the bell, but spiders could remain inside for more than a day without renewal. Spiders appeared to enlarge the bells in response to higher O 2 demands and lower aquatic P O 2 .

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.