PHOTOPERIODISM IN INSECTS AND MITES 593 



tral chain such activity remains normal); and choHnergic material 

 vanishes, reappearing only as the period of chilling is extended. The 

 implication is that the neurosecretory cells are accessible to stimulation 

 by other nervous connections, in the same manner as the neurosecre- 

 tory cells of the vertebrate hypothalamus. The temporary depolariza- 

 tion of these neurones may well be necessary to prevent neurosecretion. 



The humoral control of larval and adult (reproductive) diapause is 

 less well understood. However, there seems Httle doubt that the same 

 endocrine system, namely the brain and prothoracic glands, regulate 

 both pupal and larval diapause. But in the case of reproductive 

 dormancy the corpora allata and probably the brain are involved (de 

 Wilde, 1954). 



It will be readily apparent from these examples that the brain plays 

 a key role in the control of diapause. Unfortunately, although we have 

 valuable information as to the kind of mechanisms involved in "trig- 

 gering" the neurosecretory cells, we do not know how the brain is 

 "switched off," even in those species with an obligatory diapause. 

 There are several possibilities. One is that the mechanism is intrinsic, 

 the brain being committed from the outset to a certain "program" of 

 activity. A second is that the brain is influenced by afferent (perhaps 

 proprioceptive) nerve impulses which occur uniquely at the appropri- 

 ate point in ontogeny. However, it is not easy to visualize the steps by 

 which this input could result in the inhibition of nervous activity. A 

 third possibility is that the brain responds to a humoral stimulus, 

 linked in turn with the state of nutrition (Andrewartha, 1952; Monro, 

 1956). 



The problem is even more challenging when diapause is photoperi- 

 odically determined, since the action of this factor is often exerted 

 many instars before growth is arrested. One or more normal cycles of 

 neurosecretion (each corresponding to an instar) may therefore inter- 

 vene between the period of photosensitivity and the more immediate 

 events in the brain by which the endocrine control of diapause is estab- 

 lished. The most striking example is in the silkworm Bombyx, where 

 the Qoa type is decided by the conditions of photoperiod and tempera- 

 ture prevailing during late embryonic and early larval life (Kogure, 

 1933). 



