THE INSECTS 



139 



winter. The diapause may occur either during embryonic development 

 in the egg, or during the early part of the pupal period. The peculiar 

 physiological conditions which enable the animal to survive in a state of 

 arrest have aroused a good deal of interest, and a fair amount has been 

 discovered about them in certain cases. In the Cecropia silkworm (Lepidop- 

 tera) Williams (195 1) has shown that the diapause, which occurs in early 



Figure 8.12 



Figures a and b are dwarf pupae of the Wax moth, resulting from the re- 

 moval of the corpus allatum (source of the mouking hormone) from third 

 and fourth instar larvae; c is a normal pupa, and d a giant one produced by 

 implanting an extra corpus allatum from a young larva into one which 

 had already reached the stage at which it would normally pupate. (After 

 Piepho 1943.) e, a precocious adult of the bug PJiodnius produced by join- 

 ing a ist-stage larva to a larve undergoing the final moult; / is a normal 

 2nd-stage larva for comparison. (From Wigglesworth 1934.) 



pupal life, is under hormone control. It does not seem to be quite clear 

 how the diapause is initiated; but once the animal has passed into dia- 

 pause, it remains in that condition until the brain has been cooled for a 

 certain length of time, and then re-warmed. After this alternation of 

 temperatures, the brain is able to emit a hormone which activates the 

 prothoracic gland, and this in its turn produces a second hormone which 

 starts off the development of the pupa into the adult. The effect of the 



