1. Photoperiodic induction of diapause inNasonia vitripennis andSarcophaga argyrostoma was shown to be different in a number of respects. InN. vitripennis induction proceeds to “completion” in continuous darkness after an initial five short-day (long-night) cycles, and can be accomplished in the total absence of light by the use of daily temperature cycles, or thermoperiods.S. argyrostoma, on the other hand, requires repeated exposures to short-day (long-night) cycles for “full” induction, and thermoperiods in the absence of light are apparently ineffective. Furthermore, inS. argyrostoma periods of CO2 and N2 anaesthesia applied during the light and the dark portions of the daily cycle underlined the central importance of night-length measurement, whereas periods of chilling inN. vitripennis had previously shown light and dark to be of equal importance. 2. These results are considered to be consistent with two current models for the photoperiodic clock: internal coincidence (N. vitripennis) and external coincidence (S. argyrostoma), although none of the experimental results offersunequivocal evidence for this conclusion. 3. The apparent diversity of insect photoperiodic clocks is examined, particularly in terms of Pittendrigh's “extended circadian surfaces”. It is considered likely that the apparent diversity is a product of evolutionary divergence from a common ancestral mechanism with its basis in circadian rhythmicity.