By Francesco Tomasinelli 

Growing is a complicated process for all Arthropods, especially for big spiders like tarantulas, with their eight long legs. They can make it only by changing their skin, the exoskeleton, and assembling  a new one under the old one, before the shedding occurs.
Big spiders, like this South American tarantula, Davus fasciatus, turn on their back to begin the moulting process. Then, the “new” spiders emerges from the thorax and gently pulls out all legs form the old skin. The process can take a few minutes or hours in biggest specimens. The spider is very vulnerable in the meantime, that’s why most Artrhopods moult in a hidden retreat, here under a log. Next the blood pressure inflates the body and legs of the spider, making him grow bigger, usually of 30%. This one is a subadult specimen, so size difference is less evident.
The spider must wait some minutes or  hours (depending on size) for the new skin to harden, then slowly walks away, leaving the old useless exoskeleton.