THE NATURAL HISTORY 



OF 



AQUATIC INSECTS 



INTRODUCTION 



WATER AS A SPHERE OF LIFE 



WATER offers many advantages to the simpler 

 animals and plants. The vast extent of the seas, 

 lakes, and rivers is in itself a sufficient motive for 

 their occupation by living things, considering the 

 crowded state of the land. But more special induce- 

 ments are not wanting. The density of water supports 

 the weight of the most fragile organisms, and enables 

 them to move from place to place by such easily 

 contrived means as cilia, which may be mere drawn- 

 out threads of protoplasm, or fins, which may be 

 shaped out of a fold of skin or a row of bristles. 

 Some of the very simplest aquatic animals, such as 

 Amoeba, move along in the water by an act which 

 can only be described as flowing, the stream of living 

 protoplasm gathering itself up continually in the, 

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