Tarantulas shed their skin once in awhile to grow and this process is called moulting.
Before they moult, they will lay a bed of silk web and lie on their backs to prepare to moult. Below: a grammostola rosea preparing for its final moult They generally take abit less than a month to moult when they are 1-4cm slings and the duration between moults increase as the grow bigger. Below is a 3cm spiderling during and after it moulted. It is white even at its fangs because it has not expanded and hardened yet. After a day or two, its exoskeleton hardens and turns dark. Large Ts may take a year or 2 to moult. The more they eat the more frequent they moult and the faster they grow. Tarantulas can be most accurately sexed by their but i won't go into those details here. Large Ts only appear white on the fangs when moulting. Their colors usually become more prominent by then. Below: a Malaysian Blue Cyriopagopus sp "blue" during and after moulting. Notice the difference in color between the old skin and the new.


After moulting, their bodies are very soft and they will be very weak for several days until their exoskeletons re-harden.