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The exoskeleton is made of several layers of cuticle, a composite material containing various proteins and chitin, a long-chain polysaccharide (sugar). The chitin and protein molecules are arranged in long chains, in successive layers, like the grain in plywood.
This structure makes cuticle extremely strong, as well as highly effective at keeping the spider from drying out, but the material does have one serious drawback. While it's flexible enough for movement, it can't expand like human bones and tissue -- in other words, it can't grow. In order to increase its size, the spider has to form a new, larger cuticle exoskeleton and shed its old one (this is called molting).