Life, death and the spider skeleton
A spider (like all arthropods) has an external, cuticular skeleton provided with flexible joints. This exoskeleton encloses the blood-filled body space. Confined within this semi-rigid space the blood pressure can be varied by changes in heartbeat frequency or contraction and relaxation of muscles, notably the strong thoracic muscles. Together, cuticle and blood make up a pressurised unit known as the hydrostatic skeleton. This is important in maintaining body shape (turgor) and function. The ability to vary blood pressure is important in functions as diverse as moulting and movement. During moulting, increased heart rate results in blood pressure increases that help split open the weakened cuticle. Limb extension during movement is achieved mainly through the contraction of strong thoracic muscles which increases thoracic blood pressure and causes the limbs to extend outwards. This explains why spiders have many flexor muscles for bending their limbs inwards, but fewer extensor muscles for stretching them outwards - they're just not needed as much. It also explains why injured or dead spiders always have their legs bent inwards - they can no longer control their blood pressure and this allows the strong flexing muscles to dominate and pull the legs in under the body. This 'death position' is imitated by spiders that escape predators by dropping from a web to the ground - only this time the legs are deliberately flexed in while playing dead.
