FRESH WATER ANIMALS 205 



are scavengers. All of the other aquatic insects are predaceous, 

 mostly throughout their lives, feeding on other insects and 

 small crustaceans, sometimes on small vertebrates as well if 

 large enough. JMany of these water insects we know as fierce 

 blood suckers in their winged condition. 



After the insects the most characteristic animals of fresh 

 water are those amphibious creatures, the frogs and toads and 

 salamanders, none of which live in, and rarely even very near, 

 the sea. The greater part of these when young live in fresh 

 water; most remain near it, and some in it, all their lives. 

 Some toads and salamanders are met with far from water, and 

 some tree frogs spawn in water high above the ground in the 

 axils of bromehas or "wild pineapples" growing as epiphytes or 

 "air-plants" on the forest trees. 



Why is it that there are no insects in the sea except for a 

 single type of water strider on the surface and a few on tidal 

 beaches? Why are the toads, frogs and salamanders, and the 

 lung fishes, absent from the ocean? 



A great barrier operates to prevent migration to the sea of 

 all fresh water animals, which does not in the least aft'ect an 

 opposite migration from the sea into fresh water — the action 

 of the waves and tides. Any animal type that is in any way 

 adapted to maintain its place against the flowing action of a 

 river current or to travel or be carried overland from pond to 

 pond can pass from the ocean to fresh water. But to pass from 

 fresh water to the ocean an animal must be able to resist the 

 varying pressures of the tidal changes, and especially the ac- 

 tion of the coastal and the surface waves at their maximum 

 intensity. A degree of permanent quiet comparable to condi- 

 tions in fresh water is not found anywhere along the ocean 

 shores or out at sea at a lesser pressure than about two at- 

 mospheres. 



Insects are all air breathers. The young of water insects, like 

 the adults, have multitudes of little branching tubes within 

 their bodies which reach even to the most minute and distant 

 portions and are always full of air. Some renew the air by 



