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places. Each individual of the species, therefore, occupies at different rimes two distinct environments during its life and derives advantages from each. It is true that with some beetles, the young and the adults lire together.

FIG. 134. The nymph of a dragonfly A, the entire insect, showing the long underlip, or labium (Lb), 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 msect 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 out generalizations, furnishes the key to evolution, by which so many riddles may be solved. The grub of the bee or the wasp (Fig. 33 B) gives an excellent example of the extreme specialization in form that the young of an i*asect may take on. The creature spends its whole lire in a cell of the comb or the nest where

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INSECT