A steroid hormone, secreted by a pair of prothoracic glands in the thorax of insects and by Y organs in crustaceans, that stimulates moulting (see ecdysis) and metamorphosis. In insects its release is stimulated by prothoracicotropic hormone, and its activity is modulated by juvenile hormone (JH). Ecdysone binds to receptors in epidermal cells, and the resulting ecdysone–receptor complex triggers the expression of genes encoding proteins that digest the old cuticle and form the new one. This gives rise to the next larval stage, provided levels of juvenile hormone remain high. If levels of JH fall, then ecdysone triggers formation of a pupa and metamorphosis ensues. Some plants contain phytoecdysteroids, which are structurally similar to ecdysone. They may help to protect the plant by interfering with the moulting cycle of insect pests that consume plant tissue.