DAVIDOWITZ, G*; D’AMICO, L.J.; ROFF, D.A.; NIJHOUT, H.F.; University of Arizona; Duke University; Univ. of California, Riverside; Duke University: The physiological regulation of insect body size

Remarkably little is known about the developmental and physiological mechanisms that determine body size, or about the mechanisms by which an organism translates different environmental signals that result in plasticity of body size. We propose a model for body size regulation in the holometabolous tobacco hornworm (Manduca sexta: Sphingidae). We demonstrate that adult body size is regulated by three factors: growth rate, the timing of juvenile hormone decay as measured by the critical weight, and the timing of secretion of prothoracicotropic hormone (PTTH) and ecdysteroid secretion as measured by the PTTH delay time. We show that regulation of phenotypic plasticity of body size accentuates different components of the mechanism that regulates body size in response to different environmental stimuli. Body size increased at lower temperatures and on higher quality diets, critical weight was higher on higher quality diets but did not change with temperature, PTTH delay time increased at lower temperatures but did not change with diet, and growth rate increased at higher temperatures and on higher quality diets. Therefore, plasticity of size in response to diet quality is regulated primarily by changes in growth rate and critical weight, while plasticity of size in response to temperature is regulated primarily by changes in PTTH delay time and growth rate.