182 METABOLIC HORMONES 



Insecta. a marked decrease in respiration accompanies the 

 phenomenon of diapause, or arrested development and inertia, 

 which occurs seasonally in many insects. During diapause the 

 cytochrome c system in the cells becomes inactive, except in some 

 muscles, and the cyanide-stable respiration accounts for most of 

 the remaining low rate of oxygen consumption. The phenomenon 

 is best known in the embryos of the Japanese silkworm, Bombyx, 

 (Fukuda, 1952 and 1953) and in the pupae of the Cecropia silkworm 

 of America, Hyalophora (= Platysamia, Williams, 1952). There is 

 a certain amount of disagreement as to whether the hormonal 

 control is the same in both cases (Hinton, 1953) or different 

 (e.g. Lees, 1955). 



The action of a diapause hormone, D, secreted by the sub- 

 OESOPHAGEAL GANGLION and reducing the oxygen consumption, is 

 most clearly established in Bombyx. The situation is peculiar, as 

 compared with other examples of hormonal control, in that 

 secretion of the hormone D is determined by environmental factors 

 in one generation, but only takes effect upon the next. It appears 

 that the hormone secreted into the haemolymph of the female 

 moth passes into her ovary, where sufficient D is absorbed into the 

 eggs to put the resulting embryos into diapause within 28 hr of 

 their being laid. The eggs containing the hormone can be recog- 

 nized by their brown colour and are referred to as diapause eggs. 

 They can be made to continue their development beyond the 28 

 hr by being put into Ringer solution after being stripped of their 

 enclosing chorion and cuticle. It seems probable that this treatment 

 allows the diapause hormone D to diffuse out of the embryonic 

 tissues. 



Some races of Bombyx have only two generations in the year. 

 That w^hich develops during the long, hot days of summer lays 

 diapause eggs, which then over-winter in a dormant state. When 

 the embryos begin to develop in the short, cool days of spring, 

 they are not affected by sufficient daylight to induce the later 

 secretion of D. So they lays eggs which develop directly, without 

 diapause, during the summer, and the cycle starts again. Long 

 daylength has been found to be the most effective environmental 

 factor in determining the summer generation of these Bombyx to 

 lay diapause eggs. It acts most strongly on the females just after 



