THE PRESENT STATUS OF THE MODE OF ACTION OF INSECT JUVENILE-HORMONE

Some old and recent views concerning the mode of action of insect juvenile hormone (JH) have been reviewed. The original, 60-year-old hormone concepts were based on the qualitative, inhibitory role of JH in insect metamorphosis. After 1950, insect endocrinologists created a model of quantitative JH mode of action, on the basis of findings on epidermal transplantations in Galleria. According to this model, a single epidermal cell would produce larval characters at high concentrations of JH, pupal characters at intermediate concentrations, whereas in the absence of JH it would give rise to adult epidermal patterns. This simple model of JH action became the leading concept of insect endocrinology until this time. However, direct experimental proof that qualitative structural changes between the juvenile and adult stage are controlled by the JH concentration has never been provided. Moreover, essential aspects of the concept, such as 'reversal of metamorphosis', appear to be completely illusive. Alternative views, explaining the mode of action of JH by inhibition of morphogenesis in some generically determined ontogenetic periods have been presented. According to these views JH cannot cause the 'reversal of metamorphosis' and its action at the cellular level would proceed according to the common 'all-or-none' rule.