PHOTOPERIODIC CONTROL OF DEVELOPMENT INCHAOBORUS AMERICANUSWITH SPECIAL REFERENCE TO PHOTOPERIODIC ACTION SPECTRA

1. Responsiveness of diapausing Chaoborus americanus larvae to photoperiod increases with chilling. Both the critical photoperiod and the number of long or intermediate days required to terminate diapause decrease at the time of year when environmental photophases are rapidly increasing. Consequently, the transition from diapause-maintaining to diapause-terminating daylengths probably takes place faster in nature than is indicated by responsiveness to static photoperiods in the laboratory.2. After prolonged chilling, short days no longer maintain diapause and food alone suffices to evoke development. The switch from photoperiod-dependent to photoperiod-independent development takes place over only a ten day period.3. Energies of monochromatic light from 390 to 660 nm required to elicit 10%, 50%, and 90% development were determined by extending an otherwise white light, short-day regimen to long-day with monochromatic light at dawn or at dusk. Monochromatic light at 540 nm was most effective at evoking development both during dawn and dusk but larvae were about two orders of magnitude more sensitive during the dawn.4. Response to half hour light breaks during an otherwise short-day regimen suggests that the relative insensitivity at dusk is due to light-adaptation of the receptor pigment system during the white-light day. Greater insight into the ecology of photoperiodism will therefore probably be derived from action spectra based on the extended day technique than from those based on light breaks during the dark period.