204 ELEMENTARY STUDIES IN INSECT LIFE 



found it necessary to appropriate funds for carrying 

 on measures looking toward the permanent, reduction 

 of the numbers of this insect. The caterpillars have 

 continued to multiply and defoliate the trees each year. 

 In remedial work against this insect the State of Mas- 

 sachusetts has already spent over half a million dollars, 

 and much more must be spent before a permanent check 

 to its destructive increase can be assured. From these 

 facts it is readily inferred that the boundaries of the 

 habitat of a species are not constant, but are subject to 

 change. Every species unwittingly strives to extend 

 its territory, to gain more room to live, to disturb the 

 balance of forces which holds all in restraint. In this, 

 some species are successful, and extend their domain; 

 others are not; they cannot hold their own; they oc- 

 cupy less and less territory until they become extinct. 

 Barriers. Checks to the extension of range differ 

 with different groups of animals. Most mammals cannot 

 traverse oceans, nor can fishes cross continents. In- 

 sects, however, have a wide- distribution. This group 

 is represented in every part of the globe so far visited 

 by man, from the extreme limits of the Arctic and Ant- 

 arctic regions to the equator, and from the sea-level to 

 altitudes far above the line of perpetual snow. Hum- 

 boldt observed insects on Chimborazo at an elevation 

 of 18,000 feet. They exist in fresh and salt waters, 

 in subterranean caves and in hot springs, the waters 

 of which reach moderately high temperatures. A cer- 

 tain small fly (Psilopa] has been found breeding in 

 petroleum, a substance formerly supposed to be fatal 

 to insect life. In view of the vast numbers and varied 

 forms of insect life, it is not so surprising that this 



