An active corpus allatum (CA) in the blood feeding insect, Rhodnius prolixus, releases a substance that inhibits metamorphosis in larvae, and activates egg production in adults. In adults, transecting the nervus corporis cardiacum II's (NCCII), which are attached posteriorly to each protocerebral lobe, greatly increases egg production indicating that the adult CA is activated and receives neural inhibition from cells associated with the NCCII [Chiang, Arch. Insect. Biochem. Physiol. 39:126-131 (1998)]. In the present study, the NCCII's in fifth instar larvae were transected immediately before or after feeding to determine if these nerves normally inhibit CA activity in the last larval instar. Approximately 20 to 25 days following ingestion of a blood meal, L5's with transected NCCII's emerged as fully-formed adults with no larval characteristics. Examination of the brain in these recently emerged adults revealed that the NCCII's were absent. Since fifth instar larvae with transected NCCII's emerged with no juvenile characteristics, cutting the NCCII's did not activate the CA, indicating that the mechanism for inhibition of the CA differs in the last larval instar and adult animals.