The Influence of External Conditions 31 



The typical male and female moth of Ocneria dispar are 

 shown in Fig. i and Fig. 2. In the male the wings are gray, 

 ash, or dusky, with four zigzag black lines. The female is 

 whitish gray or slightly yellow, with the same grayish white 



FIG. 4. Ocneria dispar : male, Fig. I ; and female, Fig. 2. First generation fed 

 on walnut leaves : male, Fig. 3; female, Fig. 4. Second generation fed on walnut : 

 male, Fig. 5 ; female, Fig. 6. First generation on walnut, second on oak : male, 

 Fig. 7; female, Fig. 8. First generation on walnut, second on oak, third on wal- 

 nut, fourth on walnut : male, Fig. 9. (After Pictet.) 



pattern as the male, but more marked. The normal food is oak 

 or birch. The young caterpillars can be made to eat the leaves 

 of the walnut (Juglans regia), at first with difficulty, but the 

 subsequent generations eat it with avidity. The male of the first 

 generation is shown in Fig. 3. The wings are pale yellow, the 



