none Manduca sexta; NERVOUS-SYSTEM; Gut; NEUROTRANSMITTERS-; DIGESTIVE-SYSTEM; Gut nervous & endocrine systems, parasite effects; ENDOCRINE-SYSTEM; Gut endocrine cells, hymenopteran parasite effects; HYMENOPTERAN-PARASITES; Cotesia congregata; FMRFamide like peptides, host & parasite gut innervation & endocrine cells Cotesia congregata; NEUROTRANSMITTERS-; FMRFamide in gastric endocrine cells; DIGESTIVE-SYSTEM; ENDOCRINE-SYSTEM; Gastric endocrine cells, FMRFamide; LEPIDOPTERAN-HOSTS; Manduca sexta; FMRFamide like peptides, host & parasite gut innervation & endocrine cells Manduca sexta larvae that are parasitized by the braconid wasp Cotesia congregata enter a state of developmental arrest following emergence of the wasp larvae from the host. These fifth instar hosts linger for 2 to 3 weeks without resuming feeding, molting, or metamorphosis once the wasps emerge. Immunohistochemical staining with antiserum against FMRFamide revealed dramatic accumulation of FMRFamide-like peptide(s) in the gut nervous and endocrine systems of the developmentally arrested larvae when compared to that observed in unparasitized feeding or starved larvae. Specifically, the number of immunopositive cells and the intensity of staining was enhanced in the neurons of the frontal ganglion, the axons and axon terminals on the midgut surface, and in the gastric endocrine cells. These results were confirmed using ELISA to show that the relative amounts of FMRFamide-like peptides in midgut extracts were highly elevated in the parasitized larvae relative to the fed or starved unparasitized larvae. These data suggest that FMRFamide-like peptides in developmentally arrested larvae are produced in a significantly large number of gastric endocrine cells, and that the rate of release of the peptides may be suppressed, or the rate of their synthesis may be elevated. Localization of FMRFamide-like peptides in the gastric endocrine cells of C. congregata is also described.