DISTRIBUTION 369 



For the same reason deserts are highly effective barriers. In- 

 deed the most important checks upon distribution are those of 

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

 ful. 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 (PI. 4). The stages of an insect are frequently so 

 accurately adjusted to particular climatal conditions that an 

 unfamiliar climate deranges the life cycle. Thus many South- 

 ern butterflies find their way every year to the Northern states, 

 only to perish without reproducing their kind. Insects, how- 

 ever, are more adaptable than most other animals in respect to 

 climate, and frequently follow their food plants into new cli- 

 mates, 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 bar- 

 rier 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 indi- 

 rectly by favoring the growth of fungus diseases, to which 

 insects are much subject. 



The absence of proper food is more effective than climate, 

 as a direct check upon the spread of an animal ; food itself being, 

 of course, dependent ultimately upon climatal factors and soil. 

 Many insects, being confined to a single food plant, cannot 

 exist long where this plant does not occur ; but they will follow 

 the plant, as was just said, into new climates; thus Anosia 

 plexippus is following the milkweed over the world. The 

 2 5 



