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Adult emergence (eclosion) of fruit flies Drosophila melanogaster under constant laboratory conditions follows a circadian pattern with bouts of eclosion recurring at approximately 24 h intervals. Under periodic light:dark (LD) cycles, adults emerge only during a specific time of the day followed by little or no emergence for the rest of the day. This phenomenon is therefore equated to a gate of emergence that, when open, allows adults to emerge and when closed, no emergence takes place. In this study, we attempt to understand the mechanism underlying adult emergence rhythm in D. melanogaster using a model based on interplay between developmental and circadian clock systems. The model is composed of an oscillatory threshold of a substance that builds up during pre-adult development. Computer simulations based on this model enabled us to make specific predictions about the 'gate width' of the adult emergence rhythm under conditions of fast/slow pre-adult development and short/long circadian periods, which we subsequently tested empirically. The main predictions from the simulations are: (1) flies with faster development have greater gate width and vice versa, and (2) flies with faster circadian clocks have shorter gate width and vice versa. To empirically validate these predictions, we carried out experiments on D. melanogaster populations known to have fast/slow pre-adult development, short/long circadian periods and narrow/wide gate width. Additionally, we manipulated the rate of pre-adult development of the above flies by increasing/decreasing ambient temperature to further examine the influence of developmental rates on gate width of adult emergence rhythm by a complementary approach. The results show that gate width is greatly influenced by the duration of pre-adult development and the length of circadian cycles. This suggests that the adult emergence rhythm of D. melanogaster may be based on mechanisms involving oscillatory threshold and build-up of a developmental substance. [ABSTRACT FROM AUTHOR]