The endocrine regulation of wing polymorphism in insects: state of the art, recent surprises, and future directions

The endocrine mechanisms controlling the development and reproduction of flight-capable (long-winged) and flightless (short-winged or wingless) morphs of wing-polymorphic insects have been intensively investigated. The "classical model," put forward in the early 1960s, postulates that morph-specific differences in development and reproduction are caused by variation in the titers of juvenile hormone (JH) and/or ecdysone. Despite decades of study, the importance of these hormones in regulating wing polymorphism in aphids and planthoppers remains uncertain. This uncertainly is largely a consequence of technical and size constraints which have severely limited the types of endocrine approaches that can be used in these insects. Recent studies in wing-polymorphic crickets (Gryllus) have provided the first direct evidence that the in vivo blood titers of juvenile hormone and ecdysone, and especially the activity of the JH regulator, juvenile hormone esterase, differ between nascent morphs. Morph differences are largely consistent with the classical model, although some types of data are problematic, and other explanations are possible. Adult morphs differ dramatically in the JH titer but titer differences are more complex than those proposed by the classical model. Detailed endocrine information is thus far available only for a few species of crickets, and the hormonal control of wing polymorphism for insects as a whole remains poorly understood. Future studies should continue to investigate the role of JH and ecdysteroids in morph development and reproduction, and should expand to include studies of morph-specific differences in hormone receptors and neurohormones.