Studies on the Effect of Ecdysone-Analogues on the Development of the Silkworm, Bombyx mori L. (Lepidoptera : Bombycidae) : I. Penetration of Phytoecdysones in Larval Cuticle : The present work was supported in part by the Grant in Aid for Scientific Research from the Ministry of Education.

Abstract

The penetrability of two phytoecdysones, ecdysterone and cyasterone, through the larval cuticle of silkworm, Bombyx mori L., was tested by the dipping application of isolated abdomens of the 4th and 5th instar larvae. The two phytoecysones, dissolved in 50% ethanol, penetrated the 4th instar larval cuticle during the 4th stadium and caused the isolated abdomens to form a new larval cuticle within a few days. But the moulting hormone activity, or the penetration, decreased gradually with time after the 3rd ecdysis. Cyasterone dissolved in water was virtually ineffective.In the newly ecdysed 5th instar larvae as well, the penetration showed inducements of a supernumerous moult. However, the dipping application with phytoecdysones was ineffective on 5th instar larvae within a time as less as 24 hr after the 4th ecdysis until maturation. When 5th instar larvae approached maturation, the penetrative action of the chemical reappered and almost all abdomens isolated around the mounting stage in N₄ and Shungetsu × Hosho silkworms formed pupal cuticle. Spiracle closure and anaesthetization of the larval abdomens during the dipping application reduced remarkably the number of isolat ed abdomens responding to phytoecdysones.