136 A. D. LEES 



nation, but hardly touched on the physiological mechanisms 

 which must be involved. 



We have seen that diapause is frequently evoked by the 

 appropriate photoperiod, temperature or nutrition. It has been 

 shown that blinding fails to eliminate the photoresponse. 

 Neither the stemmata (simple larval eyes) nor the compound 

 eyes are therefore concerned. The photoreceptors have not yet 

 been identified in a diapausing insect. Nevertheless, it is certain 

 that in aphids with a similar photoperiodic reaction (which, 

 however, governs a different process — the formation of virgino- 

 parous and oviparous offspring) the receptor lies inside the 

 animal. When different areas on the head, thorax and abdomen 

 were illuminated with small spots of light for the necessary time, 

 leaving the rest of the body dark, a positive response could only 

 be elicited from the dorsum of the brain (Lees, unpublished 

 results). Whether the photosensitivity of this region is due to the 

 presence of neurosecretory cells or two other light-sensitive 

 structures (perhaps neurones), remains to be decided. 



Little is known about the role of temperature ; but a clue to 

 the possible mode of action of nutrition on diapause induction 

 is provided by the work of Johansson^^ ^j^q showed that the 

 failure of Oncopeltus to mature eggs when starving is due to the 

 inhibitory action of the brain on the corpus allatum; this 

 influence disappears when the insect is fed or the allatal nerve 

 cut. A comparable relationship in which nutrition exerts a 

 controlling influence on the activity of the brain may exist in 

 diapausing insects. 



This information, scanty though it is, suggests that the brain 

 may be the centre on which diapause-inducing stimuli act. 

 Certainly, the role of the brain in terminating diapause has been 

 proved conclusively. You will recall that unchilled cecropia 

 pupae end their diapause when supplied with a brain from a 

 chilled pupa. Davis and Schneiderman (unpublished results) 

 have demonstrated that this is indeed the site of action of low 

 temperature since brains chilled in vitro at 6° in a medium of 

 haemolymph for 8-14 weeks become competent to end diapause. 



