Book excerpt: Introduction to photoperiodism, Behavioral photoperiodism, Photoperiodism and development events, Physiological rhythms, Growth and polymorphism, Photoperiodism and diapause, Survey of photoperiodically controlled diapause, Circadian rhythms and biological time measurement, Biological clocks in developmental periodism, The dual system theory of the biological clock, Physiology of the biological clock, Photoperiodism and adaptation.