Moulting Because of the hardened skin made of chitin, the spider can not grow larger. Therefore the spider needs to shed external skeleton (exoskeleton) regularly (ecdysis) so that it can grow. The spider changes its skin 5 - 7 times in her life.

Spiders that can grow very old, like Tarantula's that may live for 25 years, change skin every year. This is not because she keeps growing but, like our clothes, the skin becomes worn out. The colour of the spider becomes darker before the changing of its skin. Enzymes dissolve the layer between the skin and the rest of the body. The new skin begins to form below the old one. It is extremely folded because it has to be larger than the old coat. The nerves stay connected to the sensory organs on her old skin so that she is not deprived of essential signals from the sensory organs on her legs. Just before shedding the old skin the spider hangs itself upside down with a thread. After several minutes the abdomen contracts to around 70% of it original size. The blood is pumped to her head raising the pressure from 20 kPa to approximately 40 kPa (0.4 atm). The skin starts tearing at her jaws and the crack enlarges to the abdomen. If the skin is loose from the head-breast part (prosoma) the blood pressure is raised in the abdomen. When the skin has become completely loose the spider falls out of her old skin. These old skins are the "dead" spiders you can see hanging to wires and small branches. Spiders can often be seen with one or more missing legs. After moulting these lost legs regenerate. After the first change of skin, newly formed legs are smaller than the original legs. After the second moulting these differences in length are hardly observable.