The role of hydraulic pressure in spider cheliceral function

Recent μCT-based surveys of spider chelicerae highlight a problematic aspect of skeletomuscular function, that is, the absence of extrinsic muscles capable of projecting the chelicerae outward or forward. Instead, all appear to generate an inward or rearward force. Previous work by comparative anatomists assigned antagonistic functions to sets of extrinsic muscles, but no anatomically distinct pivot points within a lever-based system were specified and do not appear to exist. These observations led us to revisit the problem and to propose a significant role for internal hydraulic pressure in cheliceral movement. The hydraulic hypothesis predicts that unimpeded increase in internal fluid pressure should result in inflation of basal membranes resulting in movement that could be resisted and manipulated by extrinsic cheliceral muscles. Experiments on freshly killed spiders showed that increased hydraulic pressure causes both rotational (adduction, levation, etc.) and translational (projection) movements at the cheliceral bases, movements that are synergistic with predatory strikes. This suggests that the co-option of hydraulics into cheliceral function may have increased cheliceral biting forces. Hydraulic function also allows for the basal cheliceral articulation to simultaneously produce both rotational and translational movement, a challenge for a lever-based fixed articulation. The co-option of hydraulics into additional tasks may have contributed to the evolutionary success of spiders, compared to their arachnid relatives that use hydraulics solely for locomotion.