Why do caterpillars molt as they grow?

Caterpillars, like other insects, must molt because they are encased in a rigid exoskeleton that restricts their growth. This outer covering does not expand, meaning the caterpillar can only increase in size by shedding its old skin and replacing it with a new, larger one.123

This process, known as ecdysis, is controlled by hormones. During a molt, the caterpillar produces a new cuticle beneath the existing one; once the old skin is shed, the insect can expand before the new, flexible cuticle hardens. Most caterpillars undergo this process four or five times as they grow, with each molt allowing for further development and size increase.123