324 ENTOMOLOGY 



Fresh-water streams convey incalculable numbers of insects in 

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

 withstand prolonged immersion in water, and even freezing, in man-' 

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



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

 insects, slowly but constantly. 



Birds and mammals disseminate various insects in one way 01 

 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 tl 

 San Jose scale, Colorado potato beetle, gipsy moth and many oth( 

 pests. 



Barriers.- The most important of the mechanical barriers whi< 

 limit the spread of terrestrial species is evidently the sea. Mount; 

 ranges retard distribution more or less successfully, though a speci< 

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

 break or else around one end. Mountain chains act as barriei 

 however, chiefly because they present unendurable conditions 

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

 ive barriers. Indeed the most important checks upon distribution ai 

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

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

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

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

 The stages of an insect are frequently so accurately adjusted to pai 

 ticular climatal conditions that an unfamiliar climate deranges the life 

 cycle. Thus many Southern butterflies find their way every year 

 the Northern states, only to perish without reproducing their kim 

 Insects are, nevertheless, more adaptable than most other animals 

 respect to climate, and frequently follow their food plants into ne) 

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

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



Humidity ranks next to temperature in the importance of i1 

 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 moistun 

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

 which formerly migrated eastward in immense swarms, succumbed in tl 

 moist valley of the Mississippi; the chinch bug is never seriously inji 



