On the Seasonal Dimorphism of Butterflies. 35 



nevertheless, untenable. That cold does not 

 produce the one kind of marking, and warmth 

 the other, follows from the before-mentioned facts, 

 viz. that in Papilio Ajax every generation pro- 

 duces both forms ; and, further, in the case of 

 A. Levana I have frequently reared the fourth 

 (hibernating) generation entirely in a warm room, 

 and yet I have always obtained the winter form. 

 Still, one might be inclined not to make the tem- 

 perature directly responsible, but rather the re- 

 tardation or acceleration of development produced 

 through the action of temperature. I confess that 

 I for a long time believed that in this action I had 

 found the true cause of seasonal dimorphism. 

 Both with A. Levana and P. Napi the difference 

 between the duration of the pupal period in the 

 winter and summer forms is very great, lasting 

 as a rule, in the summer generation of A. Levana, 

 from seven to twelve days, and in the winter gene- 

 ration about two hundred days. In this last species 

 the pupal state can certainly be shortened by 

 keeping them at an elevated temperature ; but I 

 have, nevertheless, only in one case obtained two 

 or three butterflies at the end of December from 

 caterpillars that had pupated in September, these 

 generally emerging in the course of February and 

 March, and are to be seen on the wing in warm 

 weather during the latter month. The greatest 

 reduction of the pupal period still leaves for this 

 stage more than 100 days. 



D 2 



