Endogenous 20-hydroxyecdysone levels in the haemolymph of non-diapause-destined and diapause-destined generations of tasar silkworm, Antheraea mylitta (Lepidoptera: Saturniidae) and associated developmental changes

A complete profile of the 20-hydroxyecdysone (20-HE) titer, development and endocrine events from 1st instar to pupation of the larvae of non-diapause-destined (NDD) and diapause-destined (DD) tasar silkworm, Antheraea mylitta Drury (Lepidoptera: Saturniidae) was studied. Diapause is induced by short days of 11 hr photophase coupled with  = 12h light, >= 26°C) and one generation is completed by August. The growth rate during the course of development of larval instars decreases and instar durations are inversely related to the body weight at the time of initiation of a larval instar. A growth compensation mechanism operates during the development of the larval instars. The growth rate was higher in early instars (1st to 4th) in both generations. The DD larvae complete the final instar in 16 days followed by a spinning stage of 13 days. The NDD larvae complete the final larval instar in 9 days followed by spinning stage of 6 days and spend 14 days in the pupal stage. The signal to release the prothoracicotropic hormone (PTTH) is related to critical body weight of larvae. From 1st to 4th instar, pre-ecdysial peaks of 20-HE were recorded in both NDD and DD generations. The programme for undergoing diapause was initiated during 3rd instar and induced by a sudden decrease in the level of 20-HE in the DD generation. Two peaks of 20-HE are required for the larval-pupal transformation, first at the wandering stage and the second at cuticle formation.