592 PHOTOPERIODISM IN INVERTEBRATES 



ENDOCRINE CONTROL OF DIAPAUSE 



Future work on photoperiodism will undoubtedly profit from recent 

 research on the regulation of insect diapause by hormones. It now 

 seems probable that humoral controlling mechanisms participate in all 

 forms of diapause whether embryonic, larval, pupal, or adult. But it 

 is equally clear that the mechanism of control, indeed the endocrine 

 systems themselves, differ according to the stage of arrest. 



Diapause in the silkworm Bombyx is controlled maternally, the type 

 of Qoo laid by the moth being decided by a secretion from the sub- 

 oesophageal ganglion (Fukuda, 1951; Hasegawa, 1952, 1957). If this 

 hormone is liberated into the blood, diapause eggs are formed in the 

 ovaries; if it is not secreted, they become nondiapause eggs. But there 

 is also a further element in the system of control: the suboesophageal 

 ganglion is in turn controlled by the brain, which inhibits the liberation 

 of the "diapause hormone" in moths previously determined by photo- 

 period and temperature as nondiapause egg producers, while permit- 

 ting or stimulating its liberation in diapause Qgg producers. This in- 

 fluence is exerted through the circumoesophageal commissures; when 

 these connectives are cut during the pupal stage, the moth will lay 

 diapause eggs regardless of the direction of the initial determination. 

 It is not yet certain whether this controlling system also operates in 

 other insects with diapause eggs. 



It is well known from the researches of Williams that the pupal 

 diapause in the silkworm Platysamia is due to the failure of the neuro- 

 secretory cells in the brain to supply the hormone necessary for the 

 tropic activation of the prothoracic glands. Without the secretion from 

 these glands, moulting and metamorphosis remain permanently in 

 abeyance. In this instance the brain controls the prothoracic glands 

 through a diffusible hormone. 



After the brain has become inactive at the onset of diapause, pro- 

 longed exposure to moderately low temperatures (ca. 6-1 5 °C) is re- 

 quired before the competence of the neurosecretory cells to secrete 

 hormone is restored (Williams, 1956). Recent work by van der Kloot 

 (1955) has shown that the dormant condition of these cells in the dia- 

 pausing insect is associated with the inactivation of the entire brain. 

 All spontaneous electrical activity ceases (in other ganglia of the ven- 



