1. A study of the course of pupal development in two strains of Drosophila melanogaster has been made in an attempt to establish the factors affecting the time of adult eclosion.

2. The time taken to complete three stages of pupal development has been measured for pupae entering each stage at each particular hour of the day when insects were kept in 12 hr. light:12 hr. darkness, 12 hr. bright light: 12 hr. dim light, or in continuous darkness.

3. The duration of each stage, in both strains, is affected by the time of day, relative to the light cycle, at which the stage is entered. The duration of each stage for pupae kept in continuous darkness is affected by the time of day at which the stage is entered, relative to the light cycle to which they had been exposed as larvae.

4. The time-interval curves for all three stages of any one strain take the same form.

5. Because of the very wide range of developmental rates, dependent upon the time within the light cycle at which each stage begins, a population in which the larvae all pupate within a 24 hr. period will continue to produce adult flies over several days.

6. The eclosion rhythm is a population effect and does not reflect the phasing of individuals to a dawn eclosion; the majority of adults emerge at dawn because of the summation effect of circadian rhythms of development at earlier stages.

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