none Spodoptera littoralis; CORPORA-ALLATA; HORMONES-; JH-synthesis by corpora allata; Inactivation by hymenopteran parasite larva & effect on host metamorphosis; METAMORPHOSIS-; Precocious metamorphosis induction by hymenopteran parasite; HYMENOPTERAN-PARASITES; Chelonus inanitus; Host precocious metamorphosis induction by parasite; Inactivation of host corpora allata by parasite larva Chelonus inanitus; LEPIDOPTERAN-HOSTS; Spodoptera littoralis; Host precocious metamorphosis induction by parasite; Inactivation of host corpora allata by parasite larva The solitary egg-larval parasitoids of the genus Chelonus induce in their lepidopterous hosts precocious onset of metamorphosis followed by developmental arrest in the precocious prepupa. The parasitoid larva emerges as a freshly moulted 3rd instar larva from the precocious prepupa. In the Chelonus inanitus Spodoptera littoralis system, we recently showed by using both direct and indirect approaches that polydnavirus and venom induce developmental arrest in the prepupa but do not induce precocious onset of metamorphosis (Soller and Lanzrein, J. Insect Physiol., in press). Here we show results of parasitoid removal experiments and of experiments where parasitoid larvae were implanted into polydnavirus and venom containing larvae (in the absence of these components implanted parasitoids were encapsulated within a few hours). All our data indicate that the presence of a late first instar parasitoid larva is a prerequisite for precocious induction of metamorphosis in the 5th stadium. Under experimental conditions, the parasitoid larva often also induced the formation of miniature 6th instar larvae, which have a smaller head capsule width and reach a lower weight than nonparasitized larvae, suggesting that modification of threshold size for metamorphosis by the parasitoid larva is a gradual effect. Measurements of juvenile hormone biosynthesis by the corpora allata in vitro revealed that in parasitized larvae the corpora allata become precociously inactivated at the moult to the 5th stadium. Implantation of late first instar parasitoid larvae into 4th instar polydnavirus and venom containing larvae led to inactivation of the recipient's corpora allata. These results suggest that in this parasitoid-host system factor(s) from the late first instar parasitoid larva inactivate the host's corpora allata which in turn leads to a precocious onset of metamorphosis.