Insects grow exponentially from hatching to adult. This growth is punctuated by periodic molts during which the exoskeleton, including the large airways of the respiratory system, is enlarged and replaced. Major tracheae increase in size after molting, fixing gas exchange capacity within an instar. Therefore, I hypothesize that larvae may become hypoxic at the end of each instar. One regulator of responses to hypoxia is the oxygen-sensing protein complex hypoxia-inducible factor 1 (HIF-1). In hypoxia, HIF-1α and HIF1β dimerize to form the HIF-1 complex, a transcription factor that controls expression of hypoxia-responsive genes. To test my hypothesis, I measured gene expression of HIF-1α and HIF-1β across various stages of larval development in the tobacco hornworm, Manduca sexta. As predicted, levels of HIF-1α and HIF-1β increase within an instar and decrease after molting. Understanding normal development of insect respiratory systems is important, because insects are crop pests and critical pollinators.