ADAPTATIONS 



311 



forth in the water so as to maintain currents to bring fresh 

 water in contact with them. Young mosquitoes (Fig. 187) 



do not have gills, but come 

 up to the surface to breathe. 

 The larvae, or wrigglers, 

 breathe through a special 



FIG. 181. The mole cricket (Gryttotalpa), 

 with fore feet modified for digging. 



FIG. 185. A water-beetle (HydropJi- 

 ilus). 



tube at the posterior tip of the body, while the pupae have 

 a pair of horn-like tubes on the back of the head end of 

 the body. 



260. Degree of structural change in adaptations. While 

 among the higher or vertebrate animals, especially the 

 fishes and reptiles, most remarkable cases of adaptations 

 occur, yet the structural changes are for the most part ex- 

 ternal, never seriously affecting the development of the 

 internal organs other 

 than the skeleton. The 

 organization of these 

 higher animals is much 

 less plastic than among 

 the invertebrates. In 

 general, the higher the type the more persistent and un- 

 changeable are those structures not immediately exposed 



FIG. 186. Wood-boring beetle larva (Prionus). 



