Is this it?
After egg hatches, the immature insect feeds and grows. Its exoskeleton is like a shell. Eventually, the larva or nymph must shed its unyielding overcoat to continue its development.
The exoskeleton which serves as its external backbone is used for protection and support. Without an exoskeleton, the insect could not survive. An old exoskeleton is shed when a new one is ready underneath, a process that can take days or weeks.