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.
Two hormones control the transitions of the moth from a juvenile into an adult. One of these hormones, called juvenile hormone, is produced and released from endocrine cells within structures called corpora allata, located at the back of the brain.
The other developmental hormone, called ecdysone, is produced and released from the developing animal's prothoracic gland. Although juvenile hormone is continually released in the larva, ecdysone is released episodically.
Ecdysone release is triggered by the presence of another hormone, called PTTH (prothoracicotropic hormone). PTTH is produced by cells in the brain, which is why it has also been called "brain hormone." PTTH is transported to and stored in paired structures called the corpora cardiaca attached to the brain. After appropriate stimulation, PTTH is released and diffuses through the extracellular fluid to the prothoracic gland, stimulating it to release ecdysone. Ecdysone diffuses to target tissues and stimulates molting.
The levels of both ecdysone and juvenile hormone control the development of the moth. Early on, the levels of juvenile hormone are high, but steadily decline and disappear by the time the animal reaches the pupal stage. In contrast, ecdysone levels peak once during every larval instar.
Ecdysone circulates in the insect and becomes activated. Its primary effect is on the epidermis, the layer of living tissue just inside the exoskeleton. The hormone triggers epidermal cells to secrete enzymes that loosen their connection with the old exoskeleton, allowing it to be shed. Then the epidermal cells synthesize a new, larger exoskeleton. In other words, a molt occurs. Each peak of ecdysone triggers a molt. Before this molt, the larva grows until it is nearly too big for its tight exoskeleton. After this molt, the first instar larva has become a second instar larva.
In addition to triggering a molt, ecdysone also promotes metamorphosis. However, high levels of juvenile hormone have the opposite effect and inhibit metamorphosis. Thus, after each molt during which juvenile hormone is present, the larva remains a larva, albeit a bigger larva.
When the larva reaches the fifth instar stage, its levels of juvenile hormone have declined dramatically. These low levels of juvenile hormone are not able to inhibit metamorphosis. The larva becomes a pupa. During the pupal stage, the animal undergoes metamorphosis and then emerges as an adult.
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 brain hormone. The brain periodically releases brain hormone 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.