Experiments on the measurement of day-length as a basis for photoperiodism were made with a stock of the carabid Pterostichus nigrita from the Subarctic in Swedish Lapland (64–66° N). Results were compared with those of earlier investigations on a Central European stock (51° N). P. nigrita from Lapland reacted differently from the Central European ones in experiments with abnormal photoperiods (cycle durations from 12 to 72 h; light period in all cases 8 h). Whereas, in the Central European stock, short-day induced reactions (previtellogenesis in the females, maturation of the male gonads) could only be observed if those cycles represented multiples of 24 h in whole numbers, ‘short-day” reactions in the Lapland stock were inducible with all cycles in nearly 100% of the specimens investigated (only in LD 8:4 was this percentage slightly diminished). Experiments with “dark breaks” of 2 h in extreme long-day (LD 20:4) revealed nearly 100% short-day maturation, irrespective of the temporal location of these dark breaks in beetles from the Lapland stock, whereas in Central European beetles only dark breaks during certain scotophile phases had such an effect. In the Central European P. nigrita, two hour light breaks during the night phase of an LD 14:10 had a destructive effect on short-day maturation processes; in the Lapland stock, these were possible in spite of 1 h light breaks during the night of an LD 19:5. In the Lapland stock, developmental processes which in the Central European stock are confined to short-day conditions could occur in all types of experiments, provided that about 3 to 4 h of darkness were given during each 24 h cycle, whether uninterrupted or not.
Light break experiments revealed that the long-day process for ovarian development (vitellogenesis) is induced in the same way as with the Central European P. nigrita. It occurs if light falls into a photosensitive phase during the second half of the 24 h cycle.
Thus, for the Subarctic population, a model is proposed in which short-day is measured by means of an hour-glass timer, whereas long-day is perceived by means of a circadian oscillator.