On the Seasonal Dimorphism of Butterflies, i 7 



furnishes a fourth generation of pupae, which, after 

 hibernation, emerge in April, as the first brood of 

 butterflies in the form Levana. 



I frequently placed pupae of this fourth brood in 

 the hothouse immediately after their transforma- 

 tion, and in some cases even during the caterpillar 

 stage, the temperature never falling, even at night, 

 below 12 R., and often rising during the day to 

 24 R. The result was always the same : all, or 

 nearly all, the pupae hibernated, and emerged the 

 following year in the winter form as perfectly pure 

 Levana, without any trace of transition to the 

 Prorsa form. On one occasion only was there a 

 Porima among them, a case for which an explana- 

 tion will, I believe, be found later on. It often 

 happened, on the other hand, that some few of the 

 butterflies emerged in the autumn, about fourteen 

 days after pupation ; and these were always Prorsa 

 (the summer form), excepting once a Porima. 



From these experiments it appeared that similar 

 causes (heat) affect different generations of A. 

 Levana in different manners. With both summer 

 broods a high temperature always caused the 

 appearance of Prorsa, this form arising but seldom 

 from the third brood (and then only in a few in- 

 dividuals), while the greater number retained the 

 Levana form unchanged. We may assign as the 

 reason for this behaviour, that the third brood has 

 no further tendency to be accelerated in its develop- 

 ment by the action of heat, but that by a longer 



c 



