1. The time taken for Drosophila melanogaster to complete three stages of pupal development has been measured for pupae entering each stage at each particular hour of the day in cycles of 12 hr. light:12 hr. darkness, 4 hr. light:20 hr. darkness, 18 hr. light:6 hr. darkness. Similar measurements have been made for insects in which the larvae were subjected to the light cycles but the pupae remained in constant darkness.
2. The duration of each stage is affected by the time of day, relative to the light cycle, at which the stage is entered. When pupae are in constant darkness the developmental rate of each stage is affected by the time of day at which the stage is entered relative to the particular light cycle to which the larval stage had been exposed.
3. The eclosion rhythm, which arises as a summation effect of rhythms of development at earlier stages, may become bimodal in light cycles with suitable photofractions.
4. The rate of development of a pupa entering a stage during the light period is related to the time-interval since the light-on signal; the preceding dark period has no effect.
5. The developmental rate of pupa entering a stage during the dark period is affected by the time-interval since the light-off signal, but may also be affected by the previous light-on signal although there is no simple relationship between them.
6. As the developmental rates are maintained in constant darkness it is concluded that the rate of development is affected by factors following a diurnal rhythm. The form of the rhythm is determined by both light-on and light off signals, but the timing of the rhythm is determined by the two signals acting independently of each other.