Juvenile Hormone and the Physiological Basis of Insect Polymorphisms

Juvenile hormone (JH) is a developmental hormone of insects. It has become evident in recent years that JH is involved in the control of gene switching and that it exerts this control only during certain critical periods. Such JH-sensitive critical periods occur periodically during the life cycle of an insect. As a rule one or more critical periods exist during each pre-reproductive instar. The presence or absence of JH during any given critical period somehow causes the insect to "choose" between alternative developmental pathway. Using this model for the action of JH, we discuss the endocrine control of metamorphosis, phase determination in locusts and aphids, caste determination is social insects, and certain color polymorphisms.