The act of molting is a usually stressful process that invertebrates go through to grow. Since they have a hard exoskeleton they literally have to break free of their old skin, revealing a new skin underneath. At the time of molting the new skin is still soft and pliable and once blood is pumped to all of the extremities, the animal becomes larger.
In the case of tarantulas this is a fascinating process. Not only do they grow larger, but any hairs that were flung off in self-defense are replaced. If a leg has broken off, this too is replaced. After a molt, the tarantula’s colors are also very bright and fresh. Unfortunately though, this is when the animal is most vulnerable as it cannot defend itself.
Tarantulas usually go on a feeding fast before molting. Even specimens that are heavy eaters will often stop altogether as they prepare. You will also usually notice the abdomen darkening and oftentimes the tarantula will web itself up into its enclosure for protection. This helps to keep away any potential predators but it also helps to increase the humidity, which is very important.
The tarantula will then make a silk pad for it to lay on. This pad is soft and covers the substrate. Once the tarantula is free of its old skin, the new skin is very soft and will tear easily. This silk pad helps to prevent this from happening by forming a barrier between the spider and the ground. Once the pad is formed the tarantula turns upside down and the process begins.