138 A. D. LEES 



Recent observations by Way^^' ^^ on the fly Leptohylemyia 

 are particularly interesting in this connection. The eggs of this 

 species exhibit two optima for diapause termination, at approxi- 

 mating +2° and — 20°. As the latter temperature is probably 

 incompatible with biochemical activity, Way has suggested that 

 the physical rupture of a lipoprotein membrane — perhaps even 

 some element of the neurosecretory cells — may be involved. 



Histological methods have as yet contributed rather little 

 towards unravelling these events. In some species, however, 

 promising results have been achieved. The hawk moth Mimas is 

 a case in point. There it has been possible to correlate the 

 chilling process with the synthesis of neurosecretory material 

 and to trace its passage down the axons to the corpus cardiacum^^' . 



RESPIRATION AND METABOLISM 



Diapausing insects are characterized by a low level of metab- 

 olism. In the cecropia silkworm., for example, a precipitous fall 

 in respiration takes place as the larva pupates. Throughout the 

 months of diapause oxygen consumption continues at a very low 

 level, only to increase dramatically as the prothoracic gland 

 hormone is secreted^^. 



Bodine and his co-workers many years ago drew attention to 

 the possibility that diapause is associated with some change in 

 the respiratory enzymes, particularly the cytochrome system. 

 This was indicated by the insensitivity of their material — the egg 

 of Melanoplus — to cyanide. This view has since been confirmed 

 and greatly extended by Williams and his associates who showed 

 that the low diapause metabolism of the cecropia moth is also 

 unaffected by cyanide and carbon monoxide. Since these in- 

 hibitors combine with cytochrome c oxidase, this result was 

 thought at one time to indicate that electron transfer proceeded 

 by a diff'erent pathway, the terminal oxidase being a flavoprotein 

 or perhaps an auto-oxidisable cytochrome b. 



Spectroscopic studies of the cytochromes in the pupal tissues 

 have shown that cytochrome b-^ and cytochrome oxidase are 



