The development of insects strongly depends on the photoperiodicity and temperature cycles of the surrounding environment. The double photoperiodic control in connection with the daylength and the interrelation between inductive and spontaneous processes are discussed as fundamental features for the physiology of photoperiodism. With his book the author proposes a new concept for a physiological basis of insect development. "The overall contribution of the book resides in its offering a series of concepts that can be discussed and tested. The ideas originating from Zaslavski's unique viewpoint should be of interest to those concerned with the evolution of life histories." (The Quarterly Review of Biology)