Ecdysteroids and juvenile hormones (JHs) are key hormones that are responsible for insect molting and metamorphosis. JH maintains the larval state and the decline of its level in the hemolymph is crucial to elicit transformation to the pupal stage; therefore, the precise control of JH biosynthesis is necessary for normal development and the initiation of metamorphosis. This chapter summarizes mechanisms of the regulation of JH biosynthesis by the corpora allata and shows that several factors such as ecdysteroids, neurotransmitters, and peptides act together in the stage-specific manner to guarantee the accurate production of JH in each stage, in particular, in the last larval stage when metamorphosis is initiated with the transformation of the larva to the pupa. In addition, recent progress in understanding the JH signaling pathway is briefly discussed, including the identification of a long elusive JH receptor.
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