130 A. D. LEES 



Lees: I do not think that genetic material is involved. In the 

 silkworm Bombyx the late embryos are light-sensitive and the 

 effect is finally seen in the adult moth. Thus although a long 

 time interval is involved, it is still the same individual and no 

 inherited effect need be invoked. In Mormoniella the photo- 

 periodic experience of the mother is transmitted to her fully 

 grown larval offspring. But in both cases the effect is confined 

 to one generation. In the next generation the switching mecha- 

 nism can again be operated in either direction by the appro- 

 priate environmental conditions. 



Nachmony: Is photoperiod only effective in inducing diapause 

 or also in breaking it? 



Lees: In some cases it is effective in both. One example is the 

 leafhopper Stenocranus which aestivates in summer under long 

 day conditions but emerges from diapause in autumn in 

 response to shorter daylengths. A second generation is then 

 produced. The moth Dendrolimiis becomes dormant as a larva 

 in autumn and then feeds and grows very little until the day- 

 length has lengthened beyond the critical photoperiod in spring. 

 Nevertheless, such examples are scarce. The diapause-inducing 

 and diapause-terminating factors are usually quite different. In 

 most species it is presumably an adaptive advantage to have 

 separate timing mechanisms governing these two events, since 

 it is unlikely that the optimal dates for diapause induction and 

 termination will have the same daylength. 



Hestrin: Could you tell us more about the fly Polypedilum 

 you mentioned? How was its remarkable resistance cO desicca- 

 tion discovered? 



Lees: It was first collected by F. L. Vanderplank from 

 shallow rock pools near Kaduna in Nigeria. During the rainy 

 season these pools are full of water and the larvae can be 

 readily observed. During the dry months the pools contain 

 nothing but a small cake of dried mud. Most entomologists, I 

 think, would pause to wonder what had happened to the 

 insects. This problem was solved when some mud was soaked in 



