The blood titres of eclosion hormone in Manduca sexta indicate that the hormone is liberated into the blood during a span of about 20 min and then disappears with a half-life (t½) of about 45 min. Eclosion normally follows 2.5 h after the appearance of the hormone in the blood.

Bursicon appears in the blood within 2 min after the newly emerged moth comes to rest at a wing spreading site. Hormone is apparently released during the following 10 min and then disappears with a t½ of 40-50 min.

The target tissues for the two hormones become responsive during the last day of development. The wing epidermis and nervous system appear to become sensitive to the eclosion hormone at about the same time, approximately 4 h before the hormone's release. Wings become responsive to bursicon about 2 h earlier.

During the last day of adult development, Manduca show a precise onset of responsiveness to bursicon and eclosion hormone followed a few hours later by the gated release of the two hormones. This appearance of hormone sensitivity is likely to be due to a photoperiodically gated event which occurs late in adult development but prior to eclosion hormone release. The nature of this event is unknown.

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Present address: School of Biological Sciences, University of Bath, Claverton Down, Bath, BA2 7AY, United Kingdom.

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