The diapause duration is programmed by the brain and can readily be transferred to other pupae by brain transplantation. This mechanism apparently functions in synchronizing the initiation of adult development among individuals that pupate at different times. Observations with field-reared hornworms are consistent with the model: September hornworms receive more short-day cycles than August hornworms and thus have a shorter diapause. Such manipulations consistently support a model in which diapause duration is inversely related to the number of short-day cycles received. The number of short-day cycles the hornworms receive can be varied by partial starvation and the use of different rearing temperatures. Hornworms transferred from long-to short-day conditions at later stages of larval development enter diapause at a lower rate, but the resulting diapause is of greater duration.

Short daylength throughout embryonic and larval development yields a high diapause incidence, but a diapause of short duration. Daylength monitored by the embryos and larvae of the tobacco hornworm, Manduca sexta, is used to program both the incidence and duration of pupal diapause.

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