Growing out of your skin
1. The old exoskeleton loosens and splits away from the new soft one underneath it.
2. Lying on one side, the spider gradually pulls its legs free.
3. The freshly moulted spider sits beside its old skin. Its new exoskeleton will take several hours to harden.
Spiders grow by periodically shedding their semi-rigid external 'skin' or cuticle of semi-rigid chitin and replacing it with a new larger skin they have grown underneath the old one (this is called ecdysis or moulting). This process includes replacement of every hair on the body and internal cuticular structures such as the fore and hind gut linings and female genitalia. Once they are mature most araneomorph spiders stop moulting. However, female mygalomorph spiders moult throughout life, although at longer intervals as they get older. This allows some species, particularly tarantulas (Theraphosidae), to grow very large.
During the moulting process, which may take many hours, the spider is in great danger from predators, being soft-bodied and unable to move until its new cuticle has hardened. For this reason moulting is often done at night hanging safely in mid-air from a silk line; or sealed within a silk retreat or moulting chamber, e.g., in a curled leaf, a burrow, or under bark.
