Having an exoskeleton presents a problem as far as growth is concerned. To function as an exoskeleton, the insect's body wall must be relatively rigid, but if it is rigid, it cannot expand very much. Therefore, as the insect grows or increases in size, the exoskeleton must be periodically shed and replaced with a larger one. The process of digesting portions of the old cuticle and synthesizing the new cuticle is called molting (also spelled moulting), which culminates in the shedding of the old cuticle (ecdysis).

The molt involves not only the cuticle of the body wall but also the cuticular linings of the tracheae, foregut, and hindgut and the endoskeletal structures. The tracheal linings usually remain attached to the body wall when it is shed. The linings of the foregut and hindgut usually break up, and the pieces are passed out through the anus. The tentorium usually breaks into four pieces, which are withdrawn through the tentorial pits during the molt. The cast skins, called exuviae, often retain the shape of the insects from which they were shed.

The initial stages in the molting cycle are triggered by the release of PTTH (brain hormone) from neurosecretory cells in the brain. This stimulates the prothoracic glands (also sometimes called the molting glands) to release ecdysone into the hemolymph. Ecdysone, in tum, stimulates the separation of the old cuticle from the underlying epidermis, a process known as apolysis. The epidermis undergoes mitosis and grows in size; after this, the new cuticle is produced. Molting fluid secreted from the epidermal cells contains enzymes that digest the old endocuticle (but do not affect the epicuticle or exocuticle), and as a new cuticle is being de posited, the digestive products are resorbed into the body. Once this new exoskeleton is complete, the in sect is ready to shed or break out of the old one. Ecdysis is triggered by a molting hormone, and begins with a splitting of the old cuticle along lines of weakness, usually in the midline of the dorsal side of the thorax. The rupturing force is pressure of the hemolymph (and sometimes air or water), forced into the thorax by con traction of the abdominal muscles. This split in the thorax grows, and the insect eventually wriggles its way out of the old cuticle.

When it first emerges from the old cuticle, the in sect is pale in color, and its cuticle is soft. Within an hour or two, the exocuticle begins to harden and darken. During this brief period, the insect enlarges to the size of that instar, usually by taking in air or water. The wings (if present) are expanded by forcing hemolymph into their veins. The alimentary tract often serves as a reservoir of the air used in this expansion: If the crop of a cockroach, for example, is punctured with a needle, the insect does not expand but collapses; if the wing tips of an emerging dragonfly are cut off, hemolymph escapes from the cut end and the wings fail to expand. In addition to allowing the cuticle to expand, this period between ecdysis and hardening of the cuticle allows insects that pupate in the soil, for example, to crawl to the surface, there to expand the cuticle. In some species, researchers have identified a proteinaceous hormone, bursicon, that controls the process of sclerotization.

The number of molts varies among most insects from 4 to 8, but some of the Odonata undergo 10 or l2 molts, and some of the Ephemeroptera may undergo as many as 28 molts. A few hexapods, such as the entognathous orders, silverfish, and bristletails, continue to molt after reaching the adult stage, but winged insects neither molt nor increase in size once the adult stage is reached. (Mayflies have a sexually immature winged instar preceding the adult, the subimago, that molts.)

The stage of the insect between ecdyses is generally called an instar. The first instar is between hatching and the first larval or nymphal molt; the second instar is between the first and second molts; and so on. However, the full process of molting is not instantaneous. There is a period of time, usually short, but sometimes very long, between apolysis and ecdysis during which the next instar of the insect is "hidden" within the old cuticle. Hinton (1971) suggested that the term instar be used to refer to period of time from one apolysis to the next, and he proposed the term pharate ins tar to refer to the insect during the time between apolysis and ecdysis. In many cases this time period is sufficiently short that little confusion arises concerning which event signals the end of one instar and the beginning of the next. However, in some, such as the cyclorrhaphous Diptera, the distinction is important. In these flies, larval-pupal apolysis is not followed by an immediate ecdysis. Instead, the last larval cuticle is hardened to form a sort of cocoon within which lies the pharate pupa. Full development of the pupa is followed by the pupal-adult apolysis. The adult cuticle is then formed, and at that point ecdysis occurs with the adult fly shedding both the last larval and pupal cuticle at the same time.

The increase in size at each molt varies in different species and in different body parts and can be influenced by a number of environmental conditions. In many insects, however, the increase generally follows a geometric progression. The increase in the width of the larval head capsule in Lepidoptera, for example, is often a factor of 1.2 to 1.4 at each molt (Dyar's rule). In species where the individual molts are not actually observed, Dyar's rule can sometimes be applied to head capsule measurements of a series of different-sized larvae to estimate the number of instars.

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الانسلاخ Moulting or Ecdysis