PHYSIOLOGY OK [NSECT DIAPAUSE. XIV. AN ENDOCRINE MKCHAXISM K()K THE PHOTOPERIODIC CONTROL OF PUPAL DIAPAUSE IX THE OAK SILKWORM, ANTHERAEA I'KKXYI CARROLL M. WILLIAMS 3 AND PERRY L. ADKISSOX -' The Biological Laboratories, l/urt'urd I'ltit'crsit}'. (<;//>//</</< 3X, Massachusetts During the final ten days of larval life, the Pernyi silkworm envelops itself in a stout-walled cocoon within which it pupates. I )evelopment may stop right there as the pupa begins a prolonged period of pupal diapause which persists until the following spring. Alternatively, the newly formed pupa may develop into an adult moth without any delay. The moth is then committed to the reproduction of a further generation of pupae which can begin to diapause before the first killing frost. If winter arrives before the larvae can pupate, the insect will experience what is little short of "ecological suicide." Like so many plants and animals, the Pernyi silkworm minimizes these eco-logical dangers by monitoring seasonal signals of utmost precision, namely, the lengths of the night and day. For ./. pcrnyl the phenomenon is well documented in the detailed studies of the Japanese investigator, Tanaka ( 1950a, 1950b, 1950c ; 1951a. 1951b : see Lees, 1955, for English summary). Thus, when Pernyi larvae are reared under day-lengths longer than 14 hours, they develop without any pupal diapause ; at temperate latitudes, photoperiods of this sort are peculiar to late spring and early summer when the season is propitious for a second brood. By contrast, larvae reared under day-lengths shorter than 14 hours (as in late summer and autumn) transform into diapausing pupae. Little is known about the physiological basis of the photoperiod response. In principle, the minimal mechanism must include, not only a photoreceptor. but also a clockwork-computer which counts the hours of darkness and daylight. Until recently, all that was known was Tanaka's finding that the larval ocelli are not involved in the reception of photoperiod. But the induction of diapause is only half the story. < >t equal significance is the termination of diapause its timing and synchronization with the seasons. In a related species, the Cecropia silkworm, the termination of pupal diapause is known to be controlled primarily by environmental temperature. By an ap-parently direct action on the brain itself, environmental temperature conditions the secretion of a hormone prerequisite for the termination of diapause and the initiation of adult development (Williams. 1 ( >5(>). This same temperature-sensitive system has generally been presumed tor other diapausing pupae including ./. pcniyi. Indeed, as pointed out in de \\ ilde's 1 This study \vas supported, in part, hy a grant from the Xational Science Foundation. -Special Postdoctoral Fellow of the U. S. Public Health Service during the term of the study. Permanent address: Department of Kntomology, Texas A & M University, College Station, Texas. 511
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