The time of pupal ecdysis in Manduca sexta can be accurately predicted by the appearance of external markers late in pupal development. Eclosion hormone injections resulted in premature ecdysis of staged pharate pupae. The latency between injection and initiation of ecdysis movements was inversely correlated with dosage and served as the basis for a sensitive, rapid, and quantitative bioassay for the hormone.

Behavioural responsiveness to the hormone arose about 8 h before normal ecdysis at 23 °C. When insects were injected at these early times the larval cuticle was not sufficiently digested to allow its rupture and these animals remained permanently trapped inside the larval cuticle.

Measurement of eclosion hormone in the blood of pharate pupae showed the appearance of biological activity in the blood about 30 min before the onset of ecdysis.

It is concluded that pupal ecdysis is under endocrine control and that the hormone involved is likely to be the eclosion hormone.

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