HOW TO KNOW THE BUTTERFLIES 



crn portions of the United States. It appears in many differ- 

 ent forms ; eleven named varieties are now recognized in the 

 United States, and still other forms are known in Europe. 



This polymorphism is partly seasonal and partly geograph- 

 ical. Thus, to illustrate a seasonal polymorphism, in the 

 State of New York the butterflies emerging from pupae that 

 have passed the winter are of two distinct forms : first, P. napi 

 virginiensis, which is smaller than the other, and with more 

 delicate wings, which are always white below ; and second, 

 P. napi oleracea, a larger form, with stronger wings, which 

 are usually tinged with yellow below. These two forms also 

 differ in the remarkable fact that oleracea gives rise to other 

 broods which appear later in the season, and are of a differ- 

 ent form, while virginiensis is single-brooded. The summer 

 broods, the offspring of oleracea, represent a third form, P. 

 napi cruciferarum, which is larger than the spring form from 

 which they have descended, with thinner wings, which are 

 of a purer white on the upper side. 



In the extreme North and in the far West other forms of 

 this species occur which differ so greatly from our common 

 forms that they were long considered as distinct species, but 

 they have been shown to be merely geographical races. 



Caterpillar. — Two-thirds of an inch long ; body small 

 cylindrical, and downy. In color it is green finely dotted 

 with black except along the back. 



Food-plants. — Cabbage, horse-radish, and other Cruciferae. 



Evidently this species has not concluded 

 whether it will in its final form be all white ; or 

 have the front margins and tips of the front wings 

 blackish ; or have one spot on each front and 



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