324 ENTOMOLOGY 



Fresh-water streams convey incalculable numbers of insects in all 

 stages; and insects as a whole are very tenacious of life, being able to 

 withstand prolonged immersion in water, and even freezing, in many 

 instances, while they can live for a long time without food. 



The universal process of soil-denudation must aid the diffusion of 

 insects, slowly but constantly. 



Birds and mammals disseminate various insects in one way or 

 another, while the agency of man is, of course, highly important. In- 

 tentionally, he has spread such useful species as the honey bee, the silk- 

 worm and certain useful parasites; incidentally he has distributed the 

 San Jose scale, Colorado potato beetle, gipsy moth and many other 

 pests. 



Barriers. The most important of the mechanical barriers which 

 limit the spread of terrestrial species is evidently the sea. Mountain 

 ranges retard distribution more or less successfully, though a species 

 may spread along one side of a range and sooner or later pass through a 

 break or else around one end. Mountain chains act as barriers, 

 however, chiefly because they present unendurable conditions of 

 climate and vegetation. For the same reason deserts are highly effect- 

 ive barriers. Indeed the most important checks upon distribution are 

 those of climate, and of climatal factors temperature is the most power- 

 ful. Tropical species cannot, as a rule, survive and reproduce in 

 regions of frost; most of the tropical species which have entered the 

 United States are restricted to its narrow tropical belts (Plate IV). 

 The stages of an insect are frequently so accurately adjusted to par- 

 ticular climatal conditions that an unfamiliar climate deranges the life 

 cycle. Thus many Southern butterflies find their way every year to 

 the Northern states, only to perish without reproducing their kind. 

 Insects are, nevertheless, more adaptable than most other animals in 

 respect to climate, and frequently follow their food plants into new 

 climates, as in the case of the harlequin cabbage bug, which has pushed 

 north from the tropics to Missouri, southern Illinois and Indiana. 



Humidity ranks next to temperature in the importance of its 

 influence upon the distribution of organisms, but in the case of animals 

 acts for the most part indirectly, by its effects upon vegetation. Thus 

 the effectiveness of an arid region as a barrier is due chiefly to the lack of 

 vegetation in consequence of the lack of moisture. Excessive moisture, 

 on the other hand, may act as a barrier. The Rocky Mountain locust, 

 which formerly migrated eastward in immense swarms, succumbed in the 

 moist valley of the Mississippi; the chinch bug is never seriously injur- 



