. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 182 S. D. BECK, I. I'.. COLVIN \\l> I). !•.. SWINTON secretory rliythni is teui]»>rallv adjusted through tlic insect's response to tlie lights- ot'l stimulus provided bv the extrinsic photoperiod. The proctodeal secretory rhythm is not. in itself, a cireadian rhythin. because il displa\s a period approximating S rather ti.an 24 liours. It is phase-set (temporally adjusted) bv tin- 24-hour rhythm of daylight and darkness, and is most certainly a component of the in>ect's photoperiodism. The ])ossible significance of

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. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 182 S. D. BECK, I. I'.. COLVIN \\l> I). !•.. SWINTON secretory rliythni is teui]»>rallv adjusted through tlic insect's response to tlie lights- ot'l stimulus provided bv the extrinsic photoperiod. The proctodeal secretory rhythm is not. in itself, a cireadian rhythin. because il displa\s a period approximating S rather ti.an 24 liours. It is phase-set (temporally adjusted) bv tin- 24-hour rhythm of daylight and darkness, and is most certainly a component of the in>ect's photoperiodism. The ])ossible significance of the effects of lights-on and lights-off stimuli on S-hour physiological rhythms in the control of growth processes was discussed in an earlier paper ( P>eck. Q. HOUR 16 20 FIGURE 7. Temporal distribution of proctodeal secretory cycle Stages 0 and 1 (solid circles) among diapan>ing larvae of the European corn borer held in a long-day photoperiod. Data such as those of Figures 0 and / gave rise to some serious questions concerning the actual time-course of tin- physiological rhythm. One such question pertained to an explanation of the relatively low fre»|uency of occurrence ot Stages 0 and 1 at the be^inuin^ of the scotopha^e. \ot all (or even most) of the larvae sacrificed at that time showed proctodeal tissue that was nearly devoid ot thioresceiit granules. One might expect that the incidence of Stages 0 and 1 would he much higher than was actually observed, if the population was to be considered reasonably homogeneous and if all of the larvae were responsive to the' photoperiodic stimuli. These considerations prompted us to undertake a more detailed study of the distri- bution of secretory stages during the hours immediately before and after the beginning ot the scotophase. The percentage incidence of the several arbitrary stages of secretion for some different time periods is shown in l-'i^ure S. "Midcvcle" is the 60-minute period between 4.5 and 3.5 hours

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