596 



PHOTOPERIODISM IN INVERTEBRATES 



the embryos are influenced directly, the Hght passing through the 

 rather transparent body wall and blood, or their determination is 

 controlled in some way by the physiology of the mother. 



Some information on this point can be gained by studying the 

 response to photoperiods of critical length. A photoperiod of 14Vi 

 hr or permanent darkness both provide conditions of "intermediate" 

 stimulation. Figure 2 shows that under these circumstances a small 

 proportion of the mothers produce virginoparae only or oviparae 

 only. Many more produce both types of progeny, but when these are 

 collected serially at birth (as an indication of the order of determina- 

 tion), it is soon evident that the two types are not intermingled ran- 

 domly but are produced in uniform batches of varying size. In some 

 instances there is an obvious oscillation between the production of 

 oviparous and virginoparous offspring. These findings suggest strongly 

 that the determination is under maternal control and is perhaps 

 mediated by an endocrine mechanism. The curve shown in Fig. 2 

 has been constructed on this basis and traces the proportion of 

 mothers which produce oviparae only when reared at different photo- 



100 -n 



- Q O m 9 9 »^ 9 9 9 



12 



14 I 15 16 

 14-5 



24 



Photoperiod, hr. per 24 hr. 



Fig. 2. The photoperiodic determination of virginoparous and oviparous 

 female progeny in the aphid Megoiira vicioe. Black histograms and 

 beaded curve indicate the percentages of mothers that produced oviparae 

 only; white histograms, the percentages producing virginoparae only; 

 shaded histograms, those producing female offspring of both types. 



