1. The thermal constant and the zero of the velocity curve have been determined for each of the pre-adult stages of Tortrix postvittana, Walk. The experimental methods are described. The thermal constant for the eggs is 153 day degrees C. and the zero of the velocity curve 4·80°C. The corresponding constants for the larvae are 469 day degrees C. and 8·05°C, and for the pupae 182 day degrees C. and 5·93°C.

2. The experimentally determined constants and the mean monthly temperatures have been used to calculate the peak of the flight period of the adult moths at Huonville, Tasmania, during the 1935, 1936 and 1937 seasons. In three cases out of four the calculated dates approximate very closely to the observed dates for the peak of the flight period.

3. The results of observations on the number, duration and head-widths of the larval instars are given. The majority of larvae fed on young apple leaves had four instars. A greater proportion of the larvae fed on mature apple leaves had more than four instars. The length of larval life at 25°C. is significantly longer for both male and female larvae when they are fed on mature leaves. The head width of the final instar of four-instar female larvae is significantly greater than that of the final instar of four-instar male larvae. Food quality has no apparent influence on the head width of the final instar of either male or female larvae.

4. At 25°C. the length of larval life of female larvae is significantly longer than that of male larvae when both are fed on the same food ; the same tendency is exhibited at other temperatures. The separate velocity curves for male and female larvae are not significantly different in slope. The difference between velocity values for male and female larvae at the mean temperature approaches significance.

5. Larval mortality increases rapidly at temperatures above 30°C. and 100 per cent, mortality occurs at 35°C. Starved larvae at low temperatures and high humidities show a tendency for the survival time to increase with the size of the larvae. Final instar larvae under these conditions can survive for two and a half months.