The hard skin made up of chitin the spider can not grow larger. The spider changes its skin 5 - 7 times in her life. Spider than can grow very old, like the Tarantula that may live for 25 years, the skin is changed every year. This is not because she keeps growing but, like our clothes, the skin becomes worn out. Before the spider changes its skin the color of the spider becomes.
Enzymes dissolve the connection between the skin and the rest of the body.
The new skin is forming below the old one and is extremely folded because it has to be larger than the old one. The nerve system stays connected to the sensory organs on her old skin because she would be deprived of essential signals from the sensory organs on her legs. Just before changing the spider hangs itself, head downwards, to a thread. The skin starts tearing at her jaws and the crack enlarges to the abdomen. If the skin has become completely loose the spider falls out of her old skin.