When an insect grows and develops, it must periodically shed its rigid exoskeleton in a process called molting. In place of the old tight exoskeleton, the insect grows a new loose one that provides the insect with room to grow larger. Many insect species also transform in body structure as they molt from a juvenile to an adult form—a process called metamorphosis.
Several hormones control insect molting and development. In the accompanying animation, we look at these hormones and the events in the life of the silkworm moth, Hyalophora cecropia. This insect undergoes complete metamorphosis—the radical transformation that occurs when the caterpillar develops into the adult moth.
A number of insects, including moths, butterflies, beetles, and flies, undergo complete metamorphosis. Each of these animals hatches from an egg into a larva (caterpillar, grub, or maggot), and then proceeds through several rounds of molting and growth.
Periodic spikes in ecdysone trigger insects to molt. These spikes are controlled, in turn, by PTTH. The brain periodically releases PTTH in response to cues from the environment, from other hormones, or from the insect's nervous system.
In addition to ecdysone, juvenile hormone also plays an important role in development. While juvenile hormone is abundant, it prevents ecdysone from triggering the insect to metamorphose after it molts—instead, the larva molts into another larger larva. When the levels of juvenile hormone decrease, the larva molts into a pupa, and metamorphosis begins.
Textbook Reference: Concept 35.5 The Insect Endocrine System Is Crucial for Development