a caterpillar's skin does not grow. as the caterpillar grows inside its skin, the skin becomes too tight. in order for the caterpillar to grow bigger, it sheds its too-tight skin. after the old skin is shed, there is a new, larger skin. this is known as moulting. a caterpillar will moult four to five times before turning into a pupa. each stage between moults is called an "instar".