Circadian systems, VI. Photoperiodic time measurement in Pectinophora gossypiella

Diapause (100% incidence) occurs in the moth Pectinophora gossypiella when it is exposed to 24-hour light/dark cycles involving 12 hours of red light (600 nm); only 2% occurs when the photoperiod is extended to 14 hours, again with 600-nm light. This wavelength fails to synchronize all the known circadian oscillations of the moth. These observations appear, therefore, to constitute positive evidence that the photoperiodic time measurement is not mediated by a circadian oscillation. However, it remains possible, even plausible, that the photoperiodic clock is a separate circadian oscillator coupled to light by a red-absorbing pigment. That possibility is testable. The nature of the clock-oscillator or not-remains open.