Signification differentielle des photoperiodes pour l'horloge biologique dePieris brassicae (Lepidoptera)

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Abstract
Photoperiodic insects are able to distinguish between long days and short days. In various models the long day response is considered the actively induced state. The short day response is thought to be passive, caused by failure of light to coincide with a photosensitive part of the night or failure of coincidence of constituent oscillators. The photoperiodic response curve of P. brassicae showed that diapause is induced by short days (4-14 h), and non-diapause state by several conditions (natural and non-natural): long days (16 h or more), LL, DD [L = light, D = dark] and ultrashort days (0.1 h). By reciprocal transfers of larvae between non-diapausing determining and diapause determining conditions, it was possible to estimate the differential capacity of 4 non-diapausing conditions vs. the diapausing action of LD 8:16 in decreasing sequence: LD 16:8 > LL > DD = LD 0.1:23.9. DD may be considered a neutral condition. In darkness the development seems to be determined by an endogenous program without external influence. LL, although being an aperiodic signal as DD, has a weak antidiapausing effect. The biological clock of Pieris differentiates between 2 constant conditions. The 4 non-diapausing conditions have the same effect on the development when applied during the entire larval life, but have different effects when only applied during a few days. Both ecological conditions LD 16:8 and LD 8:16 have an action on the development but in an opposite way. There was not a passive state caused by failure of another inductive photoperiod. Ultra-short days, DD and LL are without ecological meaning, but they provided information in attempts to determine the mechanism of the time measurement. The external coincidence model of Pittendrigh and Minis (1964) was the more adequate to explain the earlier results on the biological clock of Pieris. This model has to be modified to account for the differential significance of several non-diapausing conditions.