Insects show a wide range life history patterns ranging from insects that lack metamorphosis or undergo only partial metamorphosis (hemimetabolous insects) to those that show complete metamorphosis (the Holometabola). The three life stages that are seen in ancestral insect species -- pronymph, nymph and adult -- are proposed to be equivalent to the larva, pupa, and adult stages of insects with complete metamorphosis. This transition from hemimetabolous to holometabolous development has been accompanied by changes in the endocrine patterns underlying growth and metamorphosis and in the way that tissues respond to these hormonal cues. A key change in endocrine secretion has been a heterochronic advancement in the appearance of juvenile hormone (JH) during embryogenesis. This has been associated with the transformation of the transitional pronymphal stage of hemimetabolous insects into the holometabolous larval stage (Truman & Riddiford, 1999, Nature 410,447). The talk will explore implications of the changing pattern of endocrine secretion for postembryonic growth and development of imaginal tissues. An important outcome of metamorphosis has been the shift of the growth and differentiation of the adult form into the postembryonic period. This shift in the timing of imaginal growth is manifest in the formation of imaginal discs in the larva. In the ancestral condition, the formation and growth of imaginal discs appears to have been delayed until the end of larval life. A number of groups, however, now show a derived pattern in which imaginal disc formation and growth has been shifted into the early larval stages. The eye imaginal disc of the moth Manduca sexta is an example of an imaginal disc that shows the ancestral pattern of formation because it is delayed until the last larval stage. This delay is due to a tonic inhibition of eye disc formation by JH. Hence, the hormone that maintains the larval form also acts to suppress imaginal growth. In the derived condition, JH still acts to maintain the larval form but its ability to suppress the formation of particular imaginal discs is apparently lost so that they can form and proliferate in the presence of JH. The mechanism of this loss is yet to be determined. Another outcome of metamorphosis is that the patterning processes required for making the adult form have been moved out of the embryonic period into post-embryonic life. In the latter situation, these events have come under the control of ecdysteroids. We will discuss the insights that we have gained from molecular studies in Drosophila about how these embryonic patterning systems may have been "captured" by the ecdysone signaling pathway.

Index items: Manduca sexta, juvenile hormone, ecdysteroids, evolution of metamorphosis