146 ANIMAL LIFE 
forth in the water so as to maintain currents to bring fresh 
water in contact with them. Young mosquitoes (Fig. 87) 
do not have gills, but come 
up to the surface-to breathe. 
The larve, or wrigglers, 
breathe through a special 
Fig. 84.—The mole cricket (Gryllotalpa), Fie. 85.—A water-beetle (Hydroph- 
with fore feet modified for digging. ilus). 
tube at the posterior tip of the body, while the pupxw have 
a pair of horn-like tubes on the back of the head end of 
the body. 
81. 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  yie. 86.—Wood-boring beetle larva (Prionus). 
the invertebrates. In 
general, the higher the type the more persistent and un- 
changeable are those structures not immediately exposed 
