The circadian clock of fruit flies is blind after elimination of all knownphotoreceptors

Circadian rhythms are entrained by light to follow the daily solar cycle. W e show that Drosophila uses at least three light input pathways for this en trainment: (1) cryptochrome, acting in the pacemaker cells themselves, (2) the compound eyes, and (3) extraocular photoreception, possibly involving a n internal structure known as the Hofbauer-Buchner eyelet, which is located underneath the compound eye and projects to the pacemaker center in the br ain. Although influencing the circadian system in different ways, each inpu t pathway appears capable of entraining circadian rhythms at the molecular and behavioral level. This entrainment is completely abolished in glass(60J ) cry(b) double mutants, which lack all known external and internal eye str uctures in addition to being devoid of cryptochrome.