P. flavescens was investigated in open fields in a deciduous forest in an inland part of the Kanto region for 3 months from late June 2003. The sp. was estimated to be bivoltine from summer to late autumn. The size of the adults was unchanged throughout the season. The sex ratio of the population skewed towards 9. Maturity degree (MD), shown as the value of body weight divided by the cube of wing length, shifted upwards until the second half of August, after which it decreased sharply. Similarly the wing loading (WL) (calculated by dividing body weight by wing area) increased until the second half of August, and decreased from September, and in early October it was not significantly different between S 3 and 9 $. As the relationship of body temperature to ambient temperature showed no difference between mature and immature individuals, or between sexes, with both correlation coefficients and regression coefficients being large for a flyer type sp., they seemed to be easily affected by the ambient temperature.