Developmental mechanisms of body size and wing-body scaling in insects.

The developmental mechanisms that control body size and the relative sizes of body parts are today best understood in insects. Size is controlled by the mechanisms that cause growth to stop when a size characteristic of the species has been achieved. This requires the mechanisms to assess size and respond by stopping the process that controls growth. Growth is controlled by two hormones, insulin and ecdysone, that act synergistically by controlling cell growth and cell division. Ecdysone has two distinct functions: At low concentration it controls growth, and at high levels it causes molting and tissue differentiation. Growth is stopped by the pulse of ecdysone that initiates the metamorphic molt. Body size is sensed by either stretch receptors or oxygen restriction, depending on the species, which stimulate the high level of ecdysone secretion that induces a molt. Wing growth occurs mostly after the body has stopped growing. Wing size is adjusted to body size by variation in both the duration and level of ecdysone secretion.