Certain joints in the pedipalps of scorpions and sun-spiders lack extensor muscles but have elastic transarticular sclerites that store energy during flexion and return energy as elastic recoil during extension. This study quantifies the extension torque contributed by elastic recoil and hydraulic pressure in the chela (tibia–tarsus) and femur–patella joints of scorpions and the patella–tibia and tibi-tarsus joints of sun-spiders. Extension torque was measured as isolated joints were cycled through a natural range of angles and angular velocities by a computer controlled stepper motor. Resilience (efficiency of elastic energy return) of transarticular sclerites in the absence of internal fluid pressure was about 60% in scorpion joints and 80% in sun-spider joints. Elastic torque increased almost linearly with flexion angle in most joints except in the scorpion chela, where elastic torque decreased rapidly as the fully closed joint began to open, increased gradually and then decreased again near the fully open position. Hydraulic pressure contributed more to extension of pedipalpal joints of scorpions than those of sun-spiders. Our results indicate that mechanical properties of a “passive” transarticular sclerite can be changed by altering internal fluid pressure and by capitalizing on the sclerite's intrinsic viscoelasticity.
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