Arthropods are encased in a hard chitinous exoskeleton which both supports and protects the internal organs and muscles. Once hardened, the exoskeleton is incapable of growth or modification, and forms an unexpandable prison for the animal within. In order to grow and develop through the larval stages to the adult, insects (and crustaceans) must periodically shed this exoskeleton. This process is loosely termed molting and covers a number of events. These include the secretion of a new cuticle under the old, separation of old and new cuticles (apolysis), shedding of the old cuticle (ecdysis), and, following a rapid increase in size, the tanning of the new cuticle (sclerotization). Molting and the acquisition of adult characteristics are known to be under hormonal control, ecdysone and related compounds causing molting, and another hormone, juvenile hormone (JH), regulating the type of molt. Insects exhibit two distinct types of development. Holometabolous insects (moths, flies, bees, ants, etc.) pass through a series of larval stages (instars) before forming a pupa, emerging from this as an adult. Hemimetabolous insects (locusts, grasshoppers, aphids, roaches, etc.) develop regularly through a series of instars, each of which looks similar to the last, gradually acquiring more adult characteristics, until they finally molt directly to the adult, without an intermediary pupal stage.