Immediate effects of light on circadian eclosion and locomotor activity depend on distinct sensory input pathways

ABSTRACT Animals need to be able to sharpen circadian behavioural output in the adaptation to the variable environment. Light is the main entraining signal of the circadian clock, but can also directly increase alertness, locomotor activity, body temperature and heart rate in diurnal animals including humans. Thus, immediate effects of light can enhance or even overwrite circadian output and thereby mask circadian behaviour. In Drosophila melanogaster , immediate light effects are most evident as a lights-on response in two well described behavioural rhythms of the fly – the emergence rhythm of the adult insect from the pupa, called eclosion, and the diurnal rhythm of locomotor activity. Here, we show that the immediate effect of light on rhythmic eclosion depends on the R8 photoreceptor cells of the compound eyes, while the light response of locomotor activity is triggered by different light detecting cells and organs, that seem to compensate for the loss of each other.