28 THE STRUGGLE FOR EXISTENCE 



with excessive deliberation along its narrow path 

 and retiring always to the same spot. On this 

 perch it cannot be seen from below, and from the 

 sides and above seems almost or wholly a part of 

 the denuded midrib to which it clings ; more par- 

 ticularly when the leaves are in motion by the 

 wind, as they usually are on the trees on which it 

 feeds, particularly in the case of the aspen. 



That this mode of life is on the whole an advan- 

 tage to it is rendered probable from the fact that 

 there are two cases known, in which it is followed 

 very closely by caterpillars of the moth (Noto- 

 donta) feeding on the very same plant as species of 

 butterflies with this habit (one in Europe and one 

 in America) ; while the caterpillars of Basilarchia 

 employ a further device, the actual import of which 

 has been a puzzle. Very soon after birth, when it 

 has eaten but a very few swaths down the leaf, the 

 little fellow constructs a small and loose packet from 

 minute bits of leaf and other rejectamenta, loosely 

 fastened to one another and to the midrib, close to 

 but scarcely touching the eaten edge of the leaf ; 

 and as fast as the leaf is eaten, it removes this packet 

 (continually added to until it becomes about as big 

 as a small pea) farther and farther down the mid- 

 rib away from its perch, always keeping it near the 



