48 PHYSIOLOGICAL TRIGGERS 



the normal functioning of these very same cells during the intermediate larval 

 molts. 



In only one case do we have a clue to how environmental stimuli cause 

 delayed-action diapause, and this is in the embryonic diapause of the commercial 

 silkworm. In a remarkable series of experiments Fukuda (11-15) and Hasegawa 



(17) showed that the subesophageal ganglion of the moth produces a 'diapause 

 hormone' which causes the eggs in the ovaries to become 'diapause eggs,' i.e., 

 embryos developing from these eggs enter diapause at the very young germ 

 band stage. The production or liberation of this hormone by the subesophageal 

 ganglion can be inhibited by the brain as long as the subesophageal connectives 

 to the brain are intact. But it is important to note that whether the brain does 

 so inhibit the subesophageal ganglion depends on the photoperiod and temper- 

 ature experienced by the mother when she was an embryo. Thus we are back 

 where we began: the brain is the key and in this case its activity in adult life 

 is influenced by very specific happenings in embryonic life. The sins of the 

 mothers are visited upon the offspring at least until the first generation. 



Although this diapause factor for silkworm eggs has been found in the sub 

 esophageal ganglion of many Lepidoptera, contrary to the opinion of Hinton 



(18) there is no real evidence that it induces diapause in anything except silk- 

 worm eggs and the mechanism may be peculiar to egg diapause. A preliminary 

 report that this factor delayed the termination of pupal diapause in Cecropia 

 was later withdrawn (20). However, the subesophageal ganglion may play 

 some role in triggering neurosecretory activity for Marker (16) has just made 

 the intriguing observation that diurnal activity rhythms of the cockroach are 

 eliminated by removing the subesophageal ganglion and restored by implanting it. 



In summary it seems fair to state that we understand little how triggering 

 stimuli acting early in development have a delayed action in producing dia- 

 pause. All we know with certainty is that in all cases that have been studied in 

 detail the brain plays a central role. 



The second question, what turns on the brain and thus terminates diapause, 

 has proven more amenable to experimental analysis. In most insects diapause 

 can be terminated by exposing the insect to low temperature for a period of 

 weeks and then returning it to room temperature, whereupon the insect resumes 

 its development (3). In the Lepidoptera and Hymenoptera there is cogent 

 evidence that low temperature acts by rendering the insect's brain competent 

 to secrete the hormone that stimulates the prothoracic glands (48, 52, 9). This 

 action of low temperature on the brain is independent of connections between 

 the brain and the rest of the central nervous system; for if an isolated brain is 

 inplanted into a pupa, and the pupa chilled, the implanted brain is rendered 

 competent for neurosecretion (54). The biochemical meaning of the term 

 'renders competent' has not been determined, and to attack this question the 

 following rather simple experiments were performed with diapausing larvae 



