Effects of photoperiod and aging on locomotor activity rhythms in the onion fly, Delia antiqua

At photoperiods longer than 8 h per 24 h, adults of the day-active onion fly Delia antiqua showed a major peak of locomotor activity in the late photophase and also bursts of activity induced by lights-on or lights-off. At shorter photoperiods the activity peaks fused. After transfer from long photoperiods to constant darkness (DD), the rhythm free-ran, but only the major peak persisted. This suggests that only the major peak is controlled by the circadian pacemaker. At long photoperiods, the daily phase of the major peak occurred progressively later with age. As a result, the activity at short photoperiods often shifted from photophase to scotophase in old flies. The free-running period ( τ) also changed with age; τ was shorter than 24 h until 14–20 days after eclosion and thereafter became longer, but a few individuals repeated changes in τ. The phase delay of locomotor activity with age in D. antiqua would be attributable to the increase in τ.