8 NATURAL HLSTORY OF AQUATIC INSECTS 



density of the water with which they are filled, have 

 an Insect population of their own. Hot springs are 

 preferred by some, arctic snows by others ;^ some few 

 Dipterous larv^ live immersed in the juices which 

 flow from wounded trees and many in the fluids of 

 living animals. Some attach themselves to the roots 

 of submerged plants, and several bury themselves in 

 the substance of the freshwater sponge. 



The Dominance of Insects. 



The countless adaptations of Insects to the most 

 various conditions not only illustrate the extraordinary 

 elasticity of their plan of structure, but also their 

 power in competition. The profuse variations of the 

 Insect type would never have been called forth, had 

 not the type been eminently successful in the struggle 

 for existence. 



Such success may be shortly named dominance, and 

 Insects are an excellent example of a dominant class. 

 They are able to drive out their rivals, able to main- 

 tain and even to enlarge their territory in spite of all 

 competition. We cannot ascertain such qualities by 

 direct observation of the struggles which are going on 

 everywhere around us. Human experience is too 

 short and too blind for direct measurement of these 

 secular changes. But though we cannot see for our- 

 selves the turns of the slow and infinitely complex 

 struggles of the races of animals and plants, we have 

 before us the results of ages of such strife. The com- 



1 Examples are given by Kirby and Spence, Introduction to 

 Entomology, Vol. II., p. 231 (1826). 



