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 part between moults is called an instar.