The fluid pressure pumps of spiders (Chelicerata, Araneae)

Summary

The identity of the fluid pressure pumps in spiders was investigated in Filistata hibernalis through measurements of the activity of certain muscle groups, leg movements, and changes in fluid pressure within the leg. Our results indicate the cephalothorax is the site of the pressure pump responsible for leg extension and the musculi laterales are the major muscles involved in the operation of this pump. Fluid pressures in the legs averaged 5 100 N·m⁻² in resting spiders, ranged from 4 000 to 6 700 N·m⁻² in walking spiders and reached as high as 61000 N·m⁻² in startled spiders. Intra-abdominal fluid pressures were also measured and found to be much lower, ranging from 1000 to 4 000 N· m-2. These pressures are the result of activity of at least two sets of abdominal muscles, the sub-cuticular muscle sheet and the paired series of dorso-ventral muscles. We suggest the abdominal fluid pressure and the rigidity of the book-lungs attenuate pooling of the hemolymph in the abdomen when the spider is active. Finally we hypothesize that evolution of the hydrostatic skeleton in spiders has allowed a greater mass of flexor muscles to be incorporated into the legs and this in turn is an adaptation to the spider in prey capture.