Many hobbyists fear the molt because of the possibility of a molt-related death. However, this separation of the new and old exoskeletons is essential if the tarantula is to grow. The new skin is fully developed under the old skin up to several weeks before the molt; the old skin must shed to allow the new skin to be exposed, fill with fluids, and harden upon contact with the air.
A healthy tarantula should have no trouble during a molt, except possibly a broken leg, and it usually can take care of this itself. Death occasionally occurs for unknown reasons, perhaps due to an internal problem in the tarantula that we cannot detect, but since an adult tarantula probably has gone through more than a dozen molts, the process must be quite safe.
Many tarantulas turn on their backs to molt, possibly because they find it easier to pull their legs out of the old skin downward rather than upward. In American tarantulas that have rubbed the top of the abdomen bare, you sometimes can notice a darkening of the area caused by hemolymph accumulating between the old and new skins, but otherwise the only sign of impending molting may be that the spider stops feeding for a week or two. Molting generally takes place at night, in a secluded corner or under a cover, and may take just an hour or less. The old skin splits at the front and side edges of the carapace, and the tarantula simply pulls its entire body out through the opening. All chitinous surfaces are molted, which includes not only the mouthparts, but also the lining of the gut and other internal structures. The cast skin is a nearly perfect reproduction of the tarantula, as you might expect, and is helpful for sexing young tarantulas and correctly identifying species. Shed skins commonly are taken out, dried, wrapped loosely in tissue paper, and stored in labeled boxes for a record of growth.
In theory, the environment should have little to do with the success of a molt, but observations indicate that a tarantula that is kept too dry will have more chances of dying during a molt. Perhaps if the relative humidity is too low (less than 60 percent) the odds increase that the new skin will dry to inflexibility before it can be pulled out of the old skin. Regardless, help prevent problems by making sure the humidity near the molting tarantula is sufficient.