Molting (shedding or ecdysis) of the outer cuticular layer of the body is a process vital to arthropods, including insects and crustaceans. This profound endocrinological and physiological process has undoubtedly helped ensure the evolutionary success of the Arthropoda, which comprise the largest phylum in the animal kingdom. See also: Arthropoda; Crustacea; Insecta
The arthropod exoskeleton serves as a rigid attachment site for muscles and as a barrier against microbial invasion and the loss of ions and water from tissues. However, the exoskeleton is rigid and cannot expand. Thus, when an arthropod grows to a certain size, it must periodically shed its outer cuticle (the major part of the hard exoskeleton), and a new cuticle forms. Shedding of the old cuticle allows for increases in body size and other morphological changes. The overall process may occur over days or even weeks without obvious outward signs because the new exoskeleton forms underneath the old one. Only when the old layer is actually being shed does it become apparent that the arthropod has been undergoing a molting phase. Sequentially, the old cuticle (now called an exuviae) splits open, and the arthropod draws itself out. The new cuticle (which consists of an outermost epicuticle layer, a middle exocuticle, and an inner endocuticle) expands and hardens over several hours, primarily as the result of the cross-linking of proteins and chitin (a polysaccharide derived from glucose) in the exocuticle. See also: Endocrine system (invertebrate); Insect physiology; Metamorphosis
Several steroid hormones regulate the molting process. The three chief ones are juvenile hormone, ecdysone (20-hydroxyecdysone), and bursicon. In general, juvenile hormone governs what type of cuticle and internal tissues will form at the next molt. For example, if juvenile hormone is present during a critical period of a larval stage, the next stage will be larval. If juvenile hormone is absent, as it is during the last larval stage (or pupal stage in certain arthropods), the next molt will result in a change to the adult form. Ecdysone controls the molting process itself and is present before the molt to transform the tissues for shedding. Finally, the peptide neurohormone bursicon, often called the tanning hormone, regulates the expansion and hardening of the new cuticle, as well as the darkening of the cuticle to its characteristic brown/black color. See also: Ecdysone; Endocrine mechanisms; Hormone; Pigmentation