Biology and morphology of the parasitoidChelonus inanitus(Braconidae, Hymenoptera) and effects on the development of its hostSpodoptera littoralis(Noctuidae, Lepidoptera)

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Abstract
The solitary egg-larval parasitoids of the genus Chelonus have the unique effect of inducing in their lepidopterous hosts the precocious onset of metamorphosis and developmental arrest in the precocious prepupa. As a prerequisite to identifying the physiological and biochemical mechanisms underlying these developmental effects, we analyzed the morphology and various growth parameters in the course of development of Chelonus inanitus and studied the effect of this parasitoid on growth and development of its host Spodoptera littoralis. We show the first scanning electron microscopic study of the internal stages of an endoparasitic wasp from the embryo to the third instar. By including observations from microscopic and histological analyses we describe the different stages in the development of the anal vesicle and the formation of new segments and show that the anal vesicle has surface structures which were never described before. We also demonstrate that the head and mouthparts change entirely with each instar. Measurement of the length, width and head width at daily intervals reveals that the parasitoids grow extremely rapidly from day 6 (final phase of first instar) until day 12. According to our findings, developmental stages of endoparasitic larvae can be unequivocally defined on the basis of the parasitoid's length, the morphology of the head and the morphology of the abdomen or the anal vesicle, respectively. C. inanitus lay their eggs into eggs of S. littoralis, but the major effect on host development is seen much later when the parasitized larvae begin metamorphosis in the fifth instar while nonparasitized larvae do so in the sixth stadium. Our measurements of weight and head capsule width nevertheless show that the average weight of parasitized larvae is less than that of nonparasitized larvae already in the first instar and that the head capsule width is significantly smaller in parasitized larvae from the third stadium onwards. This demonstrates that subtle effects become manifest very shortly after parasitization. Furthermore, parasitization by C. inanitus inhibits testes development in S. littoralis. A comparison of parasitoid development in relation to host development under various conditions reveals that the parasitoid's growth rate varies with the age of the host egg at parasitization and that the parasitoid molts into the second instar always after the host has molted into its precocious last instar. Thus C. inanitus, on one hand, regulates its host by inducing precocious onset of metamorphosis and developmental arrest in the precocious prepupa. On the other hand, it adapts to host development by varying its growth rate and synchronizing its molt into the second instar with host development.