INSECTS 



places. Each individual of the species, therefore, occupies 

 at different times two distinct environments during its 

 life and derives advantages from each. It is true that 

 with some beetles, the young and the adults live together. 



B 



Fig. 134. The nymph of a dragonfly 

 A, the entire insect, showing the long underlip, or labium (Li), 

 closed against the under surface of the head. B, the head and 

 first segment of the thorax of the nymph, with the labium ready 

 for action, showing the strong grasping hooks with which the 

 insect captures living prey 



Such cases, however, are only examples of the general rule 

 that all things in nature show gradations; but this condi- 

 tion, instead of upsetting our generalizations, furnishes 

 the key to evolution, by which so many riddles may be 

 solved. 



The grub of the bee or the wasp (Fig. 133 B) gives an 

 excellent example of the extreme specialization in form 

 that the young of an insect may take on. The creature 

 spends its whole life in a cell of the comb or the nest where 



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