The work described in this paper forms the final part of an investigation into the biology and breeding of Pieris brassicae (L.) in captivity and concerns the larvae and the pupae.
The larvae of the Cambridge stock used in this investigation were found to pass through five instars in the course of their development at temperatures between 12·5 and 30°C. At the lower temperature, development was completed in 46·5 days and at the higher temperature in 11 days.
The average width of the head capsules in each instar was not affected by the temperature at which the larvae were reared, it showed little variation, and it never overlapped with that of the preceding or ensuing instar and, therefore, provides a certain way of determining the instar of any larva.
At 20°C., isolated larvae and larvae kept in crowded cultures completed their development in approximately the same time—19·6 and 18·8 days, respectively.
The average consumption of food during the whole larval period was determined in two experiments, in which it was found to be 1·42 and 1·29 g. of fresh leaves per g. of larva per day, respectively.
The duration of the pupal period ranged from 7·5 days at 30°C. to about 40 days at 12·5°C.
The adults showed a definite diel rhythm of emergence. When kept at a constant temperature, with a photoperiod from 6 a.m. to 10 p.m., nearly all the insects emerged during the dark period and that immediately following it—actually between the hours of 1 a.m. and 9 a.m. If the photoperiod is displaced 12 hours, the emergence is also displaced by the same amount, to correspond with the new dark period. If, instead of keeping the temperature constant, with the photoperiod 6 a.m. to 10 p.m., it is allowed to fluctuate, as it does naturally in June, the emergence is delayed and instead of occurring in darkness and the early hours of the morning as it does at a constant temperature, it takes place mainly during the morning and the afternoon. When insects, which have been reared at a constant temperature and a photoperiod from 6 a.m. to 10 p.m., are allowed to emerge at a constant temperature, in continuous light, there is very little evidence of a diel rhythm of eclosion but if the insects are kept in continuous darkness they show a definite rhythm of emergence. If the pupae are kept in constant light but the temperature is allowed to fluctuate, most of the adults emerge during the warmer period of the cycle.
Diapause in the pupa of P. brassicae is mainly determined by the photoperiod and the temperature during the larval stages. At 20°C., larvae reared in continuous darkness do not form diapause pupae; as the daily photoperiod increases, the percentage of diapause pupae formed also increases until, at a photoperiod of 12 hours, only diapause pupae are formed. Beyond this point the percentage of diapause pupae again declines until, with a photoperiod of about 18 hours, only non-diapause pupae are formed. At higher temperatures similar trends are observed but lower percentages of diapause pupae are formed at all photoperiods.
In P. brassicae there is no evidence that a short, sharply defined period of a day or two exists in the course of the life of the larvae during which the photoperiod operates to influence diapause.
Non-diapause pupae produced from larvae reared in continuous darkness and from larvae reared in long days (over 15 hours' light) appear to contain a growth-promoting hormone capable of causing the emergence of diapause pupae.