NEURO-ENDOCRINE SYSTEMS OF INVERTEBRATES 449 



fruitless. This is in sharp contrast to the condition of the brain only a day or 

 two before pupation, when spontaneous activity is easily recorded. Moreover, 

 spontaneous electrical activity returns to the brains of chilled animals a few 

 days before the visible onset of adult development. There is also a marked 

 contrast between the brain and all of the other ganglia of the central nervous 

 system which are active electrically throughout diapause (Van der Kloot, 

 1955a). The apparent inexcitability of the brain neurons during diapause is 

 accompanied by a fall in the resting potentials. During diapause, the average 

 resting potential of the brain neurons which were penetrated is only 23 ± 6 

 (s.D.) mV. This value is to be compared to the 65 i 8 (s.d.) mV resting 

 potentials of neurons in the brains of caterpillars and of developing adults, 

 as well as to the 65 ± 9 (s.d.) mV resting potentials of neurons in the thoracic 

 ganglia of diapausing pupae. Probably the measurements from the brain 

 include penetrations of the neurosecretory cells, which are large and conveni- 

 ently sited, but undoubtedly other large neurons were penetrated also (Van 

 der Kloot, 1956). 



These results suggest that the changes in electrical activity and in hormone 

 release are part of a causal sequence. Both stop just before the pupa molts and 

 electrical activity returns at just about the time when the hormone must be 

 released to start the development of the adult. It seems that the inexcitability 

 of the brain neurons — including the neurosecretory cells — causes the failure 

 of hormone release during diapause. 



The next problem is the chemical basis for the partial depolarization of the 

 brain neurons. The work of Schneiderman and WiUiams (1954) and of 

 Shappirio and Williams (1957) shows that in many diapausing tissues there is 

 a fall in the titer of cytochrome c. As a consequence, the metabolic rate falls 

 to a low level and the respiration is no longer depressed by cyanide or carbon 

 monoxide. However, the entire central nervous system — the brain included^ 

 retains a normal metabohc rate and cyanide-sensitivity throughout diapause 

 (Van der Kloot, 1955b). 



On the other hand, measurements of cholinesterase show that at the time of 

 pupation, the activity of this enzyme falls to a level below detection with the 

 methods used. This means that cholinesterase activity falls to 3 /^ (at most) of 

 its former level. Throughout diapause and chilling, cholinesterase remains 

 undetectable and only returns to a high level a few days before the onset of 

 adult development. The experimental evidence suggests that cholinesterase is 

 needed for conduction in Cecropia neurons (Van der Kloot, unpublished). 

 In short, it appears that changes in the activity of cholinesterase may account 

 for the loss of electrical and endocrine activites during diapause. 



The changes in the brain that are characteristic of diapause do not take 

 place if the brain is removed from a larva just before pupation and transplanted 

 into a brainless pupa (Williams, 1952). This result suggests that the changes in 

 the brain are brought about by the action of some other organ. It will be 



