For animals with exoskeletons -- like spiders, insects and crustaceans -- to grow larger, they need to shed their hard exterior.
The molting process -- also known as ecdysis -- can take minutes to hours to a month depending on the creature's size. Take the Goliath Bird-Eating spider, which the world's largest spider species that can weigh more than a third of a pound with its legspan reaching 30 centimeters.
Fortunately, the YouTube user connorwinters has captured the whole process in time-lapse video for you to see in less than two minutes (Note: the action really starts getting good around 0:30):
EightLegs.org states that molting for spiders is an energy intensive process, and they are vulnerable during this time. Right after shedding this layer, the spider's new exoskeleton needs to time to harden.
Theraphosa stirmi, which is a species of tarantula spider, is found in South America but some people have them as pets. As large and intimidating as the arachnid is, it is considered relatively harmless to humans.
(H/T: Huffington Post)