ENDOCRINOLOGY OF INSECT DIAPAUSE 137 



Van der Klooti-^ has shown that the humoral inactivity of the 

 brain during diapause is accompanied by a fall in the level of 

 cholinergic substances. The progress of chilling is marked by a 

 gradual rise in the titre of acetyl choline and finally by the 

 reappearance of cholinesterase and the resumption of electrical 

 activity. The virtual depolarisation of the brain neurones during 

 diapause appears to serve the purpose of 'protecting' the 

 neurosecretory cells either from possible sensory stimulation or 

 from random discharges from other neurones. 



Although the diapause-completing processes which take place 

 in response to chilling may be associated with changes in the 

 ordinary neural tissues of the brain, it may well be that the 

 primary events take place within the specialised neurosecretory 

 cells. If so, these processes must be extremely complex, as the 

 following well analysed instance illustrates. Schneiderman and 

 Horwitz^^ have shown that diapause in the hymenopterous 

 parasite MormonieJla can be terminated by chilling the larvae 

 at 5° for an adequate period. But if the chilling is interrupted by 

 periods of warming, the effect of low temperature is largely 

 undone. The chilling process is therefore reversible. A further 

 complication is that the temperature characteristics of the 

 diapause-completing process undoubtedly change with time. 

 The first stage requires moderate warmth, although diapause 

 can never be ended in these conditions. The second stage 

 requires low and the final stage moderately high temperatures. 

 Different phases in diapause completion can also be separated 

 by studying the effects of oxygen lack. It turns out that the 

 chilling process is aerobic, whilst the final high temperature 

 phase is favoured by anoxia. 



The action of low temperature can perhaps be most simply 

 seen in terms of competing reactions with different temperature 

 coefficients. The model proposed by Schneiderman and Horwitz 

 consists of a synthetic reaction opposed by an oxidative break- 

 down process. Low temperature slows down the latter reaction 

 but not the synthesis, thus permitting the brain to accumulate 

 the substance necessary for the production of the neurohormone. 



References p. 140 



