Abstract
The immigrant populations of Hyphantria cunea in Japan are variable in larval colour, ranging from light yellow to black in the last instar. In the native country (North America) a similar variaiton has been described, and temperature has been known as a factor involved However, the seasonal variation in larval colour that is found in the invading bivoltine populations is not accounted for by this factor alone. The results of laboratory experiments demonstrated that photoperiod was a main factor causing the seasonal variation in colour. At 25°C there was an abrupt shift in coloration around a photoperiod of 14 hr 45 min. Photoperiods shorter than this predominantly gave melanistic larvae while those longer than this produced a small number of such ones. A few light-coloured larvae occurred only at photoperiods of 15 or 16 hr. It was also found that a high temperature was comparable in its effect to a long photoperiod and a low temperature to a short photoperiod.
