PHOTOPERIODISM IN INSECTS AND MITES 587 



ment is for dormancy to set in at one particular time of year, namely 

 autumn. The insect may pass through two or more generations in late 

 spring and summer when long days permit uninterrupted growth; and 

 it is held in diapause in winter and early spring, thus escaping the 

 effect of short days at this time. The mulberry silkworm Bombyx mori 

 is exceptional in this respect, the overwintered eggs becoming photo- 

 sensitive almost immediately development is resumed in early spring. 

 Associated with this circumstance is a reversal in the response, long 

 days during early development causing the moths to lay diapause eggs, 

 and short days, developing eggs (Fig. B) (Kogure, 1933). Such a 

 short-day response is well suited to the two-generation pattern of 

 bivoltine races of silkworms, but it is obviously incompatible with 

 the production of more than two generations. 



An interesting variant has recently been described by Masaki 

 (1956) in some geographical strains of the cabbage moth {Barathra 

 brassicae. It seems that the same insect may exhibit alternative forms 

 of arrest: a transient "summer" diapause and a long-enduring "winter" 

 diapause. The summer type is induced by a long photoperiod, the 

 winter type by a short one. The essential difference between these 

 forms of diapause is underlined by the finding that summer dormancy 

 can be completed at higher temperatures than the winter. 



The photoresponse in insects and mites is often modified by other 

 environmental factors. Temperature independence is not noted to the 

 same extent as in similar reactions in plants or in light-controlled 

 diurnal rhythms. Indeed, temperature dependence is frequently an 

 adaptive mechanism in itself, whereby a rapidly breeding species can 

 take advantage of year to year variations in weather. In most long-day 

 species, high temperatures tend to suppress diapause, while low tem- 

 peratures have the opposite effect. In Bombyx, however, the response 

 is reversed and therefore remains in harmony with the reversal of the 

 photoreaction. 



The mode of action of temperature requires further study. Accord- 

 ing to Goryshin (1955) the photoperiod critical for the induction of 

 diapause in the moth Acronycta falls with successive increments of 

 temperature to the extent of almost 1.5 hr per 5°C. Short exposures to 

 temperatures below the threshold of development exert no influence 

 when they are made coincident with the dark phase, but low-tempera- 



