Spider skin
Like other arthropods, the spider's body is covered with a more or less rigid 'skin' or cuticle (the exoskeleton) made of protein and chitin. The spider cuticle consists of several layers, the outermost being toughest, covered with a thin surface wax layer that helps reduce water loss from the body. The cuticle provides internal attachment points for the muscles and helps in the regulation of blood pressure. While it is hard and protective externally, the cuticle must still accommodate the spider's sense organs - in the form of various types of innervated (supplied with nerves) hairs and pits, as well as the eyes. The cuticle even extends internally, lining the fore gut (mouth to stomach) and hind gut, the tracheal (breathing) tubes and the female's sperm storage organs (spermathecae).
The external cuticle is hardest on the cephalothorax (head and thorax) and limbs, apart from the softer cuticle that forms the flexible joints of the limbs and mouthparts. It is usually thinner on the abdomen, which can expand to some extent to help accommodate food reserves or developing eggs. However, to allow the spider to grow the entire cuticle must be shed periodically, a process known as moulting.
