Respiratory physiology and the initiation of molting in the tobacco hornworm, Manduca sexta

Abstract

<p>The regulation of body size is one of the fundamental unsolved problems in developmental biology. The control of body size requires a mechanism that assesses size and stops growth when a characteristic size is reached. In Manduca sexta the endocrine events that lead to a molt start when the larva reaches a critical weight. Molting is initiated by a size-sensing mechanism, but the nature of this mechanism has remained elusive. Here I show that this size sensing mechanism emerges from the changing relationship between the increasing body mass and the fixed tracheal system in each instar. As body mass increases, the demand for oxygen also increases, but the fixed tracheal system does not allow a corresponding increase in oxygen supply. Low oxygen tension reduces this critical weight and induces molting at smaller body size. Body size is regulated by a mechanism that senses oxygen limitation and triggers the endocrine events that lead to molting and metamorphosis.</p>