102 THE LIFE-STORY OF INSECTS [CH. 



interesting feature often to be noticed in the life- 

 story of double-brooded Lepidoptera. The butterflies 

 of the spring brood differ slightly but constantly 

 from their summer offspring, affording examples of 

 what is called seasonal dimorphism. All three 

 species have whitish wings marked with black spots, 

 larger and more numerous in the female than in the 

 male. In the spring butterflies these spots tend 

 towards reduction or replacement by grey, while in 

 the summer insects they are more strongly defined, 

 and the ground colour of the wings varies towards 

 yellowish. In the 'Green- veined' white (Pieris napi) 

 the characteristic greenish-grey lines of scaling beneath 

 the wings along the nervures, are much broader and 

 more strongly marked in the spring than in the 

 summer generation, whose members are distinguished 

 by systematic entomologists under the varietal name 

 napaeae. The two forms of this insect were discussed 

 by A. Weismann in his classical work on the Seasonal 

 Dimorphism of butterflies (1876). He tried the effect 

 of artificially induced cold conditions on the summer 

 pupae of Pieris napij and by keeping a batch for 

 three months at the temperature of freezing water, 

 he succeeded in completely changing every individual 

 of the summer generation into the winter form. The 

 reverse of this experiment also was attempted by 

 Weismann. He took a female of brt/oniae, an alpine 

 and arctic variety of Pieris napi, showing in an 



