Growth, endocrine events and ecdysone titre in penultimate instar of tasar silkworm Antheraea mylitta Drury in non-diapausing and diapausing generations

Endocrine events in penultimate instar of tropical tasar silkworm, Antheraea mylitta Drury and their relation with growth and prothoracicotropic (PTTH) hormone release timing in non-diapause destined (NDD) and diapause destined (DD) generations indicate that larvae ligated on or after day 3 show symptoms of moulting in non-diapausing generation and after day 4 in diapausing generation. Minimum weight required for making the brain competent to release PTTH in NDD penultimate instar was 7.605 ± 0.142 g and in DD it was 7.757 ± 0.123 g. On subsequent days after PTTH release both NDD and DD penultimate instars ligated worms showed symptom of moult in majority. In non-diapausing generation brain becomes more competent to release PTTH on day 4 resulting into moulting, however in diapausing generation it is on day 5. Ligation studies confirm that symptom of moulting represented as effects of PTTH on PG stimulation was not observed in NDD on day 3, however, on day 4 larvae showed moulting symptoms in late scotophase. Similarly, in diapausing generation a few percent of fourth instar larvae showed moulting symptoms in early photo phase of day 5. Neurosecretory cells (NSCs) in brain of IV instar larvae contained different intensity of neurosecretory material. There were different group of median neurosecretory cells (NSCs) active at different age of penultimate instar. Suboesophageal ganglion (SOG) and thoracic ganglion (TG) of non-diapausing penultimate instar were full of NSCs with PF-positive material, though these cells did not contain this material in diapausing generation. Probably, NSCs of SOG and TG in DD generation liberate their neurosecretory content for diapause programming. There is variable dependency of penultimate instar on different cues of PTTH release and initiation of moulting in non-diapausing and diapausing generations. Haemolymph ecdysone levels are generally higher during dusk in non-diapausing generation and during dawn in diapausing generation.