Photoperiodic time measurement in insects: A review of clock models

Based on analyses of responses of insects and mites to a wide range of diel and nondiel experimental light-dark schedules, a variety of models have be en developed for the photoperiodic clocks in these species by nearly as man y investigators. According to some of these models, the photoperiodic clock is based on a mechanism separate from the circadian system, that is, a so- called "hourglass." According to other models, the clock is based on one or more circadian oscillators that may be coupled to each other and that may or may not show a certain degree of damping. In this context, a rapidly dam ping oscillator could be regarded as an hourglass. The present article give s an overview of the many different clock models and their philosophies, an d it makes comparisons among them to provide a better understanding about h ow these models are related, if at all, and why the double circadian oscill ator model is the most favored model at present.