Recent studies on the rice stem borer,CHILO SUPPRESSALIS Walker,are presented as an example of some results of spatial variation in population quality.Local differences in the seasonal prevalence of this insect are basically related to the alternative occurrence of a short life-cycle strain which has a strong tendency to multivoltinism, and a long life-cycle strain which has a deep intensity of diapause along the climatic line. Qualitative change in a local population is illustrated by some effects of phase polymorphism in the armyworm,LEUCANIA SEPARATA Walker. The active black larvae that appear in crowded populations and the sluggish pale larvae that appear in sparse populations differ in their daily rhythmic activity and in their resistance to starvation at different humidities.