168 INSECT TRA.\SF0R3IATI0.\S. 



leaves, being thence protected, remain soft and pulpy. 

 But as soon as the inner leaves receive an accession 

 of sap, which rises from the roots on the return of 

 spring, their vessels swell and their nervures expand; 

 while the outer leaf, from its vessels being shrunk and 

 partly obliterated, undergoes little change besides 

 being pushed out and sometimes entirely thrown off 

 by the growth of the inner leaves, which it had pre- 

 viously enclosed. It may be remarked, also, that this 

 outer envelope of a bud is not united with the inner 

 leaves by any interlacing of their substance or of 

 their vessels, though in some cases there is an adhe- 

 sive gluten which partly binds them together; but this 

 is never so strong as to prevent the expansion of the 

 leaves. On comparing one of the bud envelopes thus 

 thrown off, we can scarcely persuade ourselves that 

 so small a covering cou'd ever have contained the 

 large spreading leaves which have burst from them. 



A caterpillar corresponos in several circumstances 

 with the leaf bud. The outer skin encloses a suc- 

 cession of several other skins each becoming more 

 delicate, soft, and indistinct than the one exterior to 

 it, but gradually, like the expanding leaves, growing 

 more substantial and firm as it receives a supply of 

 nutriment. Tlie chief mechanical difference be- 

 tween the leaves folded up in the bud and the suc- 

 cessive caterpillars enveloped within the skin of one 

 newly hatched, is that the leaves in the bud receive 

 all their nourishment through their foot- stalks from 

 the root of the tree, whereas the caterpillar is nour- 

 ished from within by the food digested in its stomach. 

 The superfluous nourishment, usually in considerable 

 quantity, and called the fat of the caterpillar, appears 

 to lie between the successive skins, in a similar way 

 to the adhesive gluten in the leaf bud. But as the 

 first inner skin expands and increases in consistence, 



