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How Insects Grow

The Molting Process

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A recently molted swallowtail caterpillar.

A recently molted swallowtail caterpillar, with the old exoskeleton.

Photo © Debbie Hadley, WILD Jersey

Insects grow in increments; in fact, all arthropods do. Each stage of growth ends with molting, the process of shedding and replacing the rigid exoskeleton. People often think molting is the simple act of an insect breaking out of its skin and leaving it behind. In truth, it's a complex process involving hormones, proteins, and enzymes.

When Molting Occurs:

Insect growth occurs in the stage immediate following egg hatch. As the immature insect feeds and grows, its exoskeleton remains an inflexible container. Eventually, the larva or nymph must shed this unyielding overcoat to continue its development.

Without the exoskeleton for protection and support, the insect could not survive. The old exoskeleton cannot be shed until a new one is ready underneath, a process that takes days or even weeks.

The Molting Process:

When there is no more room for the insect to expand inside its exoskeleton, a hormone triggers molting. The exoskeleton separates from the underlying epidermis. Molting fluid fills the newly created gap underneath. Epidermal cells secrete proteins to form a new cuticle, which serves as a barrier between the insect and the molting fluid.

With the new cuticle in place, enzymes in the molting fluid digest the inner layer of the exoskeleton. Chitin and protein are recycled by the epidermal cells, then secreted under the cuticle. This secretion becomes the procuticle, the major component of the new exoskeleton.

Once the new exoskeleton is formed, the insect can begin the familiar step of shedding its old one. A large intake of air helps the insect expand its body, and muscular contractions force the outer shell to split, usually down the dorsal side. The bug squeezes from the outgrown exoskeleton.

The insect must continue to swell and expand the new cuticle, so it is large enough to allow room for more growth. The new overcoat is soft and much paler than the former one, but over a few hours, it becomes darker and begins to harden. Within a few days, the insect appears to be a slightly larger copy of its former self.

 

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