EVIDENCE FOR DAWN AND DUSK HOURGLASSES IN PIMPLA PHOTOPERIODIC CLOCK

Resonance experiments (Nanda-Hamner protocol) conducted at two tempera tures for diapause termination in Pimpla instigator (Hymenoptera : Ich neumonidae) do not support the view that the photoperiodic clock has a n oscillatory component, but suggest the presence of a non-rhythmic ti mer or hourglass mechanism. These results are best explained by a two hourglasses model, one of which starts at light-on and measures the ph otophase and the other is initiated by light-off and measures the scot ophase. The most likely hypothesis is that the ratio of photophase to scotophase lengths is the determining element. Good agreement is obtai ned between results predicted by two hourglasses model and results obs erved in Pimpla. The diurnal hourglass continues to run for long time (several months) in constant condition (LL) and does not require to be 'turned over' by D/L transition, in contrary to the classical model o f hourglass which executes a single act of time measurement in extente d phase and then stops. The most simple explanation is that some essen tial factor of diapause termination is synthesized during photophase a nd degraded during scotophase. Therefore an independent photoperiodic counter (for sommation of daily informations) is not necessary. The tw o hourglasses system serves as photoperiodic clock and accumulation of product as counter.