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The fruit fly, Drosophila melanogaster, shows a bimodal circadian activity rhythm with peaks around light-on and before light-off. This rhythm is driven by seven groups of so-called clock neurons in the brain. To dissect the multioscillatory nature of the Drosophila clock system, the process of reentrainment to a reversed light cycle was examined by using wild-type flies and cryb mutant flies that carry a strong loss-of-function mutation in cryptochrome (cry) gene. The wild-type flies showed that the morning peak dissociated into two components, while a substantial fraction of cryb flies exhibited dissociation of the evening peak into two components that shifted in different directions. When the temperature cycle was given in constant darkness in such a manner that the thermophase corresponded to the previous night phase, the morning peak also split into two components in wild-type flies. These results suggest that both morning and evening peaks are driven by two separate oscillators that have different entrainability to light and temperature cycles. Examination of the process of reentrainment to a reversed LD in mutant flies that lack some of the four known circadian photoreceptors (compound eyes, ocelli, CRYPTOCHROME [CRY], and Hofbauer-Buchner [H-B] eyelets) revealed that these four photoreceptors play different roles in photic entrainment of the four putative oscillators. [ABSTRACT FROM AUTHOR]