The synchronisation of growth and reproduction in insect species with the favourable seasons within their range of dispersal, and of their ac tivity with the daily rhythm of light and darkness, are of extremely high survival value. Specific diurnal or nocturnal activity in insects is related to temperature and water requirements, feeding habits, and morphological ap pearance. Circadian rhythms as recently reviewed by Harker (44) are, however, beyond the scope of this article. Diapause and many forms of polymorphism in insects of the temperate zone are season-bound. Students of these phenomena have, in the past, often failed to elucidate the external responsible. Thus, Tower ( 121) in Leptinotarsa, Babcock (3, 4) in Ostrinia nubilalis (Hubner) (=Py rattsta nubilalis), Dawson (28) in Antheraea polyphemus (Cramer) (Telea polyphemus), and Steinberg & Kamensky (1 1 1) in Loxostege sticticalis (Linnaeus) did not manage to establish clear-cut associations between any of the components of the physical environment which they studied and the inception of diapause. Recent work on the photoperiodic relations of these four species has provided a sound basis for an understanding of their sea sonal activity. The same is true for the role of internal factors in diapause. The concept of maternal physiology as introduced by Simmonds ( 108) has gained importance by the rediscovery of the extensive studies of Kogure (63) on delayed photoperiodic responses of diapause in Bombyx mori (Lin naeus) and the subsequent endocrinological analyses by Fukuda (36) and by Hasegawa (48), as well as by recent work of Lees (73, 74) with the aphid M egoura viciae Buckton. To explain the timing of seasonal dimorphism in butterflies, Weismann ( 126) had to accept hypothetical inner factors, whereas Danilyevsky (15) and Miiller (89), with the classical example of Araschnia levan a (Lin naeus) (forms levana and prorsa), have demonstrated the photoperiod to be the primary causal factor. As a token stimulus correlating growth and reproduction of insects with the favourable season, photoperiod has the advantage of being an immediate expression of the astronomical determining the daily and