Circadian clocks are endogenous timing mechanisms that control molecular, cellular, physiological, and behavioral rhythms in organisms ranging from cyanobacteria to humans. These genetically determined clocks are entrained (synchronized) to the 24-hour day primarily by the daily light-dark cycle. Many aspects of insect biology are affected by circadian clock-driven rhythmic behaviors, including the timing of egg hatching, adult eclosion, reproductive behavior, social interactions, and more remarkable circadian behaviors, such as time-compensated sun-compass orientation in migratory species (Saunders 2002).