Molting

Insect cuticle is tough. Cuticle resists trauma and repels water. Apodemes or attachment points inside rigid cuticles are firm supports for muscles that move the body. Sheets and tubes of cuticle form wings and appendages. Cuticle protects delicate internal parts from predators and parasites, making insects tougher prey, but because cuticle is hard and tight like a coat of armor, and because cuticle stretches minimally, cuticles restrict continuous growth. The principle structural component of cuticle is chitin, a modified cellulose molecule. Chitin and products from the old cuticle are absorbed and probably recycled as about eighty percent of the new cuticle may contain materials from the old (Ref: Chitin).

Growth and changes in an insect's form occur only after molting or during metamorphosis after new soft cuticle has replaced the old. Two hormones, juvenile hormone and molting hormones (ecdysteroids), control molting. Rising titers of these hormones in the hemolymph drive epidermal cells to synthesize DNA and RNA in preparation for molting. If juvenile hormone is present before a critical molt, the insect retains its larval characteristics. Ifjuvenile hormone is lacking but molting hormones are present, the insect becomes adult.

Many factors regulate molting's complicated sequence. The sequence begins with turning on production of hormones that initiate molting. The same epidermal cells that grew the old cuticle let it slough in a controlled manner by first pushing the old cuticle outwards so it may be shed before the same cells deposit new, soft cuticle. Growth or expansion of the body occurs abruptly after the molt to be followed by hardening or sclerotization of the new cuticle.

The epidermis, a thin layer of cells lining the surface of the body below the cuticle, secretes the new cuticle. The cuticle varies in toughness and thickness in different regions of the body and at different stages. Before the old cuticle sheds, the epidermal cells secrete digestive enzymes into the developing space between the old and the new cuticles. Stereotyped movements, such as rotating the abdomen, free the old cuticle from the epidermis. Active transport of potassium ions and bulk flow of water into the space between the old and new cuticles produce a molting fluid that may buffer the pH changes created as the old cuticle is digested and broken down. The insect splits the old cuticle by increasing internal hydrostatic pressure and swelling its body. Some insects even swallow air filling their alimentary tracts displacing the hemolymph to expand the hemocoel that stretches the cuticle. Some insects may even pump hemolymph into the thorax expanding and stressing the old cuticle, splitting it along specific lines of weakness.

In both types of metamorphosis, as each stage grows, the cuticle around it gets too small like tight fitting trousers. This old cuticle finally splits to reveal a fully formed new cuticle around the new body. Molting ceases once an insect is fully grown. Materials and energy for renewal of the exoskeleton must all pass through the depot of the hemolymph, and recycling of metabolites and controllers within the hemocoel determines how materials distribute to the body.

The soft new cuticle covers a very soft body, so hemolymph probably serves as a hydrostatic skeleton during this vulnerable period. After the body expands and the cuticle hardens, the volume of hemolymph decreases.

Molting during metamorphosis is only one example of system wide coordination of myriad individual controllers that accomplishes a very complex task. But first, how do controllers and control systems interact?

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