On the Seasonal Dimorphism of Butterflies. 1 3 



development, it should be possible to compel the 

 pupae to take one or the other form at pleasure, by ,, 

 the application of the necessary external conditions. 

 This has never been accomplished with Araschnia 

 Prorsa. As in the experiment already described, 

 and in all subsequent ones, single specimens ap- 

 peared as the unchanged summer form, others 

 showed an appearance of transition, and but very 

 few had changed so completely as to be possibly 

 taken for the pure Levana. In some species of the 

 sub-family Pierince, however, at least in the case 

 of the summer brood, there was, on the contrary, 

 a complete transformation. 



Most of the species of our ' Whites ' (Pierince) 

 exhibit the phenomenon of seasonal dimorphism, 

 the winter and summer forms being remarkably 

 distinct. In Pieris Napi (with which species I 

 chiefly experimented) the winter form (Figs. 10 

 and 11, Plate I.) has a sprinkling of deep black 

 scales at the base of the wings on the upper side, 

 while the tips are more grey, and have in all cases 

 much less black than in the summer form ; on the 

 underside the difference lies mainly in the frequent 

 breadth, and dark greenish-black dusting, of the 

 veins of the hind wings in the winter form, while 

 in the summer form these greenish-black veins 

 are but faintly present. 



I placed numerous specimens of the summer 

 brood, immediately after their transformation into 

 chrysalides, in the refrigerator (o 1 R.), where 



