Abstract
The widely accepted assumption that insects clearly distinguish between short days and long days on the basis of a threshold mechanism does not apply in Pieris brassicae. This species is capable of measuring the length of the scotophase of each natural light-dark cycle within a dark range of 13–5 h. Changes of the scotophase in the conventional “short-day range” as well as the “long-day range” correlate with changes in the proportion of diapausing individuals. Longer scotophases generally have a stronger diapause inducing effect than shorter ones. This is clearly demonstrated by shortening the scotophase a mere 5 min. Additionally, a concept of time measurement based on Gibbs' model is discussed. It is postulated that the different inductive strengths of various scotophases are dependent on the syntheses of different amounts of an “unknown substance”. The inductive strength is influenced by temperature. The substance synthesized is accumulated daily and added up to an “induction sum”. A photoperiodic response is induced if an innate threshold of the “induction sum” is exceeded. The adaptive significance of a quantitative measurement of photoperiodic events is seen in the ability of a more flexible response to actual environmental conditions in individuals of migrating species like P. brassicae.
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