Classified within the arthropods, isopods have a chitinous exoskeleton and jointed limbs. Unlike insects, spiders have an endoskeleton in addition to their exoskeleton. The exoskeleton or cuticles consists of chitin, a polymer of glucosamine. A mantis nymph grows bigger as it molts its exoskeleton. Insect wings are adult outgrowths of the insect exoskeleton that enable insects to fly. Moulting occurs whenever a specimen outgrows its rigid exoskeleton. As they grow and mature, like all arthropods they shed and molt away their exoskeleton. Insect growth is constrained by the inelastic exoskeleton and development involves a series of molts. Ingrowths of the arthropod exoskeleton known as apodemes serve as attachment sites for muscles. They are stocky, possessing both an arthropod-like exoskeleton and a spinal-based nervous system. Lobsters are invertebrates with a hard protective exoskeleton. The insect wing is an outgrowth of the exoskeleton. Blue crabs grow by shedding their exoskeleton, or molting, to expose a new, larger exoskeleton. The new exoskeleton is initially soft but hardens after the moulting of the old exoskeleton. The cuticle sheds with every moult along with the exoskeleton.