302 ENTOMOLOGY 



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. Intentionally, 

 he has spread such useful species as the honey bee, the silkworm and 

 certain useful parasites; incidentally he has distributed the San Jose 

 scale, Colorado potato beetle, gypsy 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 effective barriers. 

 Indeed the most important checks upon distribution are those of climate, 

 and of climatal factors temperature is the most powerful.^ Tropical 

 species, as a rule, cannot 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 fre- 

 quently so accurately adjusted to particular climatal conditions that an 

 unfamiliar climate deranges the life cycle. Thus many Southern butter- 

 flies find their way every year to the Northern states, only to perish 

 without reproducing their kind. Insects, however, are 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 cab- 

 bage bug, which has pushed north from the tropics to Missouri, southern 

 Illinois and Indiana. 



Humidity ranks next to temperature in the importance of its in- 

 fluence 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, 

 migrating eastward in immense swarms, succumbs in the moist valley 

 of the Mississippi; the chinch bug is never seriously injurious in wet 

 years. Moisture checks the development of these and other insects in 

 ways as yet unascertained; possibly it acts indirectly by favoring the 

 growth of fungus diseases, to which insects are much subject. 



