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 asa 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 1n 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 
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