What accounts for the difference in the emergence times of Drosophila melanogaster between the first and second eclosion days?

One of the characteristics of the adult eclosion rhythm in Drosophila melanogaster is that adult emergence time differs greatly between the first and second eclosion days. The emergence time is in the middle of the light period on the first eclosion day, but immediately after light-on on the second day. We hypothesized that incomplete entrainment of the endogenous pacemaker to the light–dark (LD) cycle is responsible for the daily variation. Due to the very short pupal period of this species, adult emergence may occur before complete synchronization of the pacemaker with the external cycle on the first eclosion day. Therefore, the peak time on the first eclosion day may differ significantly from that on the second day. To verify this hypothesis, using pupae that had pupariated within 24 h, the time difference between the first and second peaks was compared in LD 12:12 or constant darkness at five different temperatures from 30 to 15°C. In both light regimes, the time difference decreased with decreasing temperature and extended pupal duration. The interval between the eclosion peaks approached 24 h, supporting this hypothesis. These results can be interpreted by using a two-oscillator model.