AbstractThe prothoracicotropic hormone is an insect neuropeptide released into the hemolymph to signal molting and metamorphosis through its stimulation of steroidogenesis. The only known source of the prothoracicotropic hormone in the tobacco hornworm,Manduca sexta, has been a group of lateral cerebral neurosecretory cells, the L‐NSC III. In this study, the developmental and spatial distribution of the prothoracicotropic hormone was examined throughout the life cycle ofManduca.In common with many vertebrates and invertebrates in which neuropeptides are located in several regions within the central nervous system (CNS), the prothoracicotropic hormone phenotype inManducais expressed by CNS neurons in addition to the L‐NSC III. These neurons are located in the brain, frontal ganglion, and subesophageal ganglion. One cerebral neurosecretory cell group, the ventromedial neurons, expresses the prothoracicotropic hormone phenotype and the behavioral neurohormone, eclosion hormone. Whereas the L‐NSC III and the ventromedial neurons express the peptide phenotype throughout the life cycle, the other neurons express the peptide only during the embryonic and larval stages. This precise spatial and temporal expression of the prothoracicotropic hormone by different groups of neurosecretory cells raises the possibility that inManducathe peptide may, in addition to its known neuroendocrine function, play other physiological roles in different ways at different stages of the life cycle. © 1993 Wiley‐L