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ped means of storing the mineral, since on land they cannot rely on a steady supply of dissolved calcium carbonate.[39] Biomineralization generally affects the exocuticle and the outer part of the endocuticle.[38] Two recent hypotheses about the evolution of biomineralization in arthropods and other groups of animals propose that it provides tougher defensive armor,[40] and that it allows animals to grow larger and stronger by providing more rigid skeletons;[41] and in either case a mineral-organic composite exoskeleton is cheaper to build than an all-organic one of comparable strength.[41][42] The cuticle may have setae (bristles) growing from special cells in the epidermis. Setae are as varied in form and function as appendages. For example, they are often used as sensors to detect air or water currents, or contact with objects; aquatic arthropods use feather-like setae to increase the surface area of swimming appendages and to filter food particles out of water; aquatic insects, which are air-breathers, use thick felt-like coats of setae to trap air, extending the time they can spend under water; heavy, rigid setae serve as defensive spines.[23] Although all arthropods use muscles attached to the inside of the exoskeleton to flex their limbs, some still use hydraulic pressure to extend them, a system inherited from their pre-arthropod ancestors;[43] for example, all spiders extend their legs hydraulically and can generate pressures up to eight times their resting level.[44]






Moulting Main article: Ecdysis Cicada climbing out of its exoskeleton while attached to tree The exoskeleton cannot stretch and thus restricts growth. Arthropods, therefore, replace their exoskeletons by undergoing ecdysis (moulting), or shedding the old exoskeleton after growing a new one that is not yet hardened. Moulting cycles run nearly continuously until an arthropo