Introduction

After the formation of the early embryo, most of insect development is governed by the hormonal milieu, beginning with the formation of the first instar larva within the egg. Whether maternally derived hormonal signals are also important during early embryogenesis is still unclear. Much is known about the hormones involved and the regulation of their synthesis and secretion, especially in the Lepidoptera (see Gilbert, 1989, Gupta, 1990, and Ohnishi and Ishizaki, 1990, for recent reviews), and about the role of these hormones in directing development on the organismal and cellular levels (see Riddiford, 1985, for a review). Yet how these hormones act at the molecular level is only just beginning to unfold, principally through studies on Drosophila melanogaster.

This chapter focuses on the present state of knowledge of the hormonal regulation of gene expression in Lepidoptera. It begins with an overview of the endocrine basis of molting, metamorphosis, and reproduction in Lepidoptera and a description of the mode of action of the insect developmental hormones. This is followed by a short description of the systems in which particular genes have been cloned and studies of their hormonal regulation have been initiated. At the end, focus is on the hormone receptors and their actions, in both Drosophila and Lepidoptera, and questions for the future are posed.

Hormonal control of molting and metamorphosis

Because insects have a rigid exoskeleton, they must molt and shed this skeleton in order to increase in size and to change their form at metamorphosis.