Photoperiod is the main environmental cue used by northern insects to predict the forthcoming seasonal changes and to adjust their life-history traits to fit these changes. We studied the effects of photoperiod on egg-to-adult development time, juvenile body mass and female reproductive diapause in two northern Drosophila montana populations with different patterns of voltinism. The most interesting findings were consistent between the populations: (1) when maintained before eclosion in short day conditions, representing early autumn, the flies developed faster and were lighter than when maintained in long day conditions, representing early summer, (2) photoperiodic time measurement is apparently reset after eclosion, adjusting the flies' development according to post eclosion conditions, (3) the sensitive period for diapause induction took place after eclosion and (4) there was no direct connection between females' egg-to-adult development time and their reproductive state at adulthood, which suggests that these traits can be determined by photoperiodic cues through different time measurement systems. Independence of photoperiodic regulation of life-history traits before and after eclosion enable D. montana flies to respond to changing photoperiods on a short time scale and match their life-history traits according to seasons.
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