CHAPTER V. 
I 
INFLUENCE OF METEOROLOGICAL CONDITIONS ON THE 
DEVELOPMENT AND MIGRATIONS OF LOCUSTS. 
That the increase and diminution of insects depend very largely upon 
meteorological conditions is now too well known to require further proof. 
As a very general rule, which has but few exceptions, warm, dry years 
are favorable to an increase of insect life, while cold, wet seasons have 
a tendency to diminish their numbers. Not only is this true, but the 
development is to a large degree in proportion to the heat and dryness y 
that is to say, when the season is unusually hot and the drought ex- 
cessive, insects abound and enormously exceed their ordinary numbers. 
This is especially noticeable in the case of such insects as the true- 
locusts and other Acridians, the chinch-bugs, most of the Aphides and 
many Lepidoptera. But as a general rule the maximum development 
of a species requires two successive favorable years ; at least, such is th& 
case with the locusts and chinch-bugs. 
We may state, therefore, as a proposition which we presume will be 
admitted as correct, that the development and movements of the locusts are 
very largely influenced by meteorological conditions. The extent to which 
these conditions effect their development and govern their movements 
is a point not yet fully settled, but much of the uncertainty in this 
respect has been cleared up by the investigations of the commission. 
The effect of wind, heat, cold, and moisture upon the movements of 
the locusts is so marked that it had been observed before the days 
of Pliny ; even the writers of the Old Testament show by their state- 
ments that they were aware that the wind is necessary to aid them in. 
flight. Moses states, 202 "And the Lord brought an east wind upon the 
land all that day, and all that night ; and when it was morning, the east 
wind brought the locusts ; " and again : 203 "And the Lord turned a mighty 
strong west wind, which took away the locusts, and cast them into the 
Red Sea." 
Pliny, as heretofore stated, mentions the fact that certain authors con- 
tend that they could not fly at night on account of the cold, although 
he tries to disprove this by the hypothesis that they cross the Mediter- 
ranean Sea from Africa to Italy ; yet it shows that these writers had 
observed the effect of cold upon their flights. 
A. Heat and dryness. — As already mentioned in this chapter, migra- 
tory locusts are found only in regions of more than ordinary dryness, 
which are free from the shade and moisture-retaining influence of forests y 
at least indicating, if not proving, that the migratory instinct is caused 
by, or in some way depends upon, this dry condition of the region in- 
habited. 
""Ex., x, 13. 
203 Ex., x, 19. 
109 
