ENDOCRINOLOGY OF INSECT DIAPAUSE 133 



glands was a diapausing insect, the giant silkmoth Hyalophora 

 cecropia. The combination of large size and tolerance of drastic 

 surgical procedures makes this the material of choice. 



When WiUiams removed the brains from diapausing cecropia 

 pupae he found that they failed to develop even if chilled for 

 the length of time which would normally bring dormancy to an 

 end. On the other hand, when brains from chilled pupae were 

 transplanted into unchilled pupae, development was promptly 

 initiated. The interaction of brain and prothoracic glands was 

 tested by implanting these organs, either separately or together, 

 into the isolated pupal abdomen, which is without any known 

 endocrine source of its own. Moulting and metamorphosis was 

 only caused when both organs were present. 



These results suggested that the immediate cause of diapause 

 in the cecropia pupa was the inactivity of the neurosecretory 

 brain cells, and that the failure of growth was due to the 

 absence of a promotive hormone (ecdyson) rather than to the 

 presence of an inhibitory factor. This hypothesis was tested by 

 grafting together a chilled and an unchilled pupa. Both members 

 of the pair developed, indicating that no inhibitory substances 

 of any significance were present in the unchilled insect. This 

 conclusion appears to hold good in many species. Indeed, 

 growth inhibitors of the type found in plant seeds are unknown 

 in dormant insects. 



As might be expected, the brain-prothoracic gland system has 

 also proved to be the main controlling system in larval diapause. 

 Nevertheless, it may well be that a further endocrine — the 

 corpus allatum — is sometimes involved. It is well known that 

 in larval insects this organ produces a secretion — the juvenile 

 hormone — which leads to the retention of the larval characteris- 

 tics. Unmistakable signs of activity have been found in the 

 corpora allata of some diapausing insects, for example in the 

 larvae of the rice stem borer Chilo^. However, it is still uncertain 

 whether these organs play any positive role in stabilizing 

 diapause. 



The humoral control of embryonic diapause, which has been 



References p. 140 



