An Ontogenetic Perspective on the Timing of Maturation in Insects with Special Consideration of Physical and Resource Thresholds

Abstract

All animals progress through a series of functionally discrete life stages from fertilization through adulthood and often into senescence. Reproduction in the adult stage can only occur after maturation--the final life history transition during ontogeny--from juvenile to adult. Despite a robust literature that predicts the optimal body size and development time at which this transition should occur, the ontogenetic factors that are responsible for determining metamorphosis and the manner in which they are translated into the hormonal mechanisms regulating maturation remain unresolved in most species. In this dissertation, I first review the theoretical context and importance of understanding maturation from both life history and physiological/developmental perspectives. Then I review the literature that describes various ontogenetic factors thought to determine the onset of maturation in insects. Finally, I present four studies that examine two of the major hypotheses that have been proposed to explain the onset of metamorphosis in insects using Manduca sexta larvae (Lepidoptera: Sphingidae) as a model organism. In the first study, I show that physical thresholds are unlikely to be the factor that determines maturation in larval M. sexta because larval insects do not seem to be growth constrained in the manner assumed in the literature. Next, I present three chapters that examine the possibility that attainment of a resource storage threshold is the determining factor for the onset of metamorphosis. In the first of these studies, I show that there is a hemolytic factor present after metamorphic commitment that can induce precocious metamorphosis in larval M. sexta, indicating that maturation can be coordinated at least partially from developing tissues throughout the body. The following study examines resource storage during metamorphic commitment in the final larval instar of M. sexta. I show that resource storage is of critical importance during the final period of larval growth in terms of mass allocation. Even with environmentally-induced variability in resource storage, growing M. sexta appear to have a target amount of stored resources near the body weight at which metamorphic commitment occurs. Individuals reared on reduced quality diet store proportionally more resources with a higher caloric value than individuals reared on in terms of fat growth, which is consistent with a decrease in the body weight at metamorphic commitment observed in other studies. Individuals reared at different temperatures invest differently in resource storage during growth; however, resource storage content tends to converge at the critical weight, which may explain invariance of the critical weight in response to rearing temperature found in other studies. Finally, I examine resource storage in the context of allocation tradeoffs with growth and metabolic rate. I demonstrate that storage increases substantially as growth rate declines in the final larval instar of M. sexta.