Molting
In arthropods, the shedding of the cuticle. Arthropods, such as insects and crustaceans, owe a large part of their evolutionary success to their exoskeleton. It serves as a rigid attachment site for muscles and also provides a barrier against microbial invasion from the outside and water and ion loss from within. A rigid body covering, however, is not readily expandable. Consequently, at periodic intervals a new exoskeleton is formed and the old one shed to allow increases in body size or changes in morphology. Most of this process, termed molting, is hidden since the new exoskeleton forms underneath the old one. See also Arthropoda.
Molting begins with the detachment of the epidermis from the overlying cuticle, forming a space between the two structures. Cell divisions may then occur in the epidermis followed by the initiation of secretion of the new cuticle. The epicuticle is first laid down, followed by the exocuticle. Consequently, toward the end of a molt, one finds the animal with a new exoskeleton consisting of epicuticle, exocuticle, and some endocuticle, inside the partially digested old exoskeleton. The events that surround the escape from this old cuticle can be divided into three discrete phases: preparatory, ecdysis, and postecdysis.
The preparatory phase includes the behavioral and physiological changes that lead up to ecdysis. A key physiological event is the activation of the molting fluid and the resulting digestion of the inner layers of the old endocuticle, a major store of protein and chitin. The preparatory phase also involves behavior that brings the animal to an appropriate site for ecdysis.
The ecdysial movements bring about the shedding of the old exoskeleton. These are very stereotyped behaviors that are due to central motor programs whose output is modified to a greater or lesser extent by sensory feedback as the behavior progresses. Ecdysis is often accompanied by the activation of dermal glands that pour their secretion onto the surface of the new cuticle as the old one is being shed. These secretions make up an outer cement layer which further aids in waterproofing the cuticle.
The postecdysial phase is devoted to the expansion and hardening of the new exo-skeleton. Typically, the new exoskeleton is larger than the old one, so it needs to be inflated to its proper size. After its expansion, the new cuticle is tanned and acquires its new pigmentation. In crustaceans, the new cuticle is gradually mineralized through deposition of calcium into it.
In all arthropods, molting is caused by a class of steroid hormones, the ecdysteroids, the most prominent of which is 20-hydroxyecdysone. The ecdysteroid profile during a molt is characterized by initial high titers of the steroid followed by a gradual decline in levels as the molt progresses. Activation of the enzymes in the molting fluid, the digestion of the endocuticle, and molting fluid resorption all occur as the steroid titer drops. Importantly, the decline in ecdysteroids is also necessary for the release and action of the neuropeptide, eclosion hormone, that triggers ecdysis itself. Other neuropeptides are then released as a consequence of eclosion hormone action. Bursicon is the tanning hormone, and its primary target is the epidermis. Other peptides released at this time cause the circulatory changes that direct the blood into the expanding appendages. See also Endocrine system (invertebrate).
