206 ELEMENTARY STUDIES IN INSECT LIFE 



group exists over such a vast area and under such di- 

 verse conditions. What has been said of the group as 

 a whole cannot be said of the individual species in this 

 group. They, in their attempt to extend their range, 

 are subject to barriers. The barriers which restrict 

 the spread of species of insects may be classed under 

 two general divisions : (a) barriers which prevent en- 

 trance into new territory; (6) barriers which prevent 

 maintenance after introduction. 



Barriers to Entrance. Checks, such as oceans, or high 

 mountain ranges, which affect many animal forms, are 

 not always valid in the case of insect species. Insects 

 surmount some of these obstacles through powers of 

 flight, or are transported. While it is not possible for 

 them to carry themselves across oceans or over high 

 mountain ranges, they are frequently transported dur- 

 ing quiescent periods such as the egg and pupal stages. 

 The eggs, the pupa, or even the larva, of a wood-boring 

 species, may drift in the heart of a log across an ocean. 

 Through chance, mammals or birds may carry species 

 of insects, during the quiescent stages of insect develop- 

 ment, across possible insect barriers into new territories. 



Barriers to Existence. -Introduction into new terri- 

 tory does not decide the establishment therein of the 

 insect introduced. Climate is an important element 

 in the life history of an insect. Some insects require 

 a high altitude with its accompanying dry atmosphere 

 and moderate heat. Others can exist only under con- 

 ditions to be found in low lands with their marshes 

 and humid atmospheres. The collector who studies 

 the same locality for some time will notice that the in- 

 sects dwelling in marshy meadows differ from those of 



