104 EEPORT UNITED STATES ENTOMOLOGICAL COMMISSION. 
larly ; this is especially found in the district lying hetwccn the Libyan Desert and 
Iran, between the Arabian and Persian Gulfs; in Arabistan, sbut in by it-i adjacent 
States, and, in a broader 6ense, with the regions of the Jordan and Euphrates south- 
ward toward Jemen. 
As a further evidence that all these conditions are necessary to their 
excessive increase and the development of the migratory instinct, it is 
found that P. migratorius is unable to maintain continued existence in 
Germany or Poland; and that C. sprctus cannot remain permanent on 
the prairies of Texas, Kansas, or Nebraska, or, in fact, in any portion 
of the area we have denominated the "temporary region." The same 
thing is also doubtless true in reference to the other migratory .species. 
It is apparent, therefore, that a more than ordinarily dry and rarefied 
air is necessary to the development of the migratory instinct; at least 
we are justified by the facts in assuming this. If it is argued that this 
condition is necessary to their excessive increase in numbers, and that 
this increase makes change of place necessary in order to procure food, 
ami that this is the cause of migration, we reply thai C. 8pretU8,sd Least, 
is essentially migratory in its native habitat, and that it will migrate 
whether in excessive numbers or not; and that the same fact appears 
to be true of P. migratorius. It follows, then, that the disposition to 
migrate does not depend upon numbers, but is owing to some other 
cause; for why should C. spretus and P. migratorius become so excess- 
ively multiplied under this intluence and other species under the same 
conditions not ? 
As a further proof that want of food is not only not the remote cause 
of migration, but is not always the immediate cause, we may adduce the 
now well-known fact that swarms reared in the temporary regions will 
leave the fields of wheat and barley, and the rich grass of the prairies, 
to return to the barren plains from which their ancestors came. It is 
also a fact that those reared iu the mountain canons, where there is 
abundant food, even though there be but few compared with the supply 
of food, will, when they have reached the proper age, fly away with the 
first favorable wind. Nor is this only an occasional occurrence but a 
constant habit; in fact it is almost universally the case. Both on the 
plains and in the mountains, even when there is abundant food at hand, 
often when the day is warm and clear and no wind is blowing, they may 
be seen circling upwards, evidently desiring, as shown by their uneasy 
movements, to move away, only waiting for the air to assist them. 
These facts, which are brought out more fully elsewhere, I think show 
conclusively that the desire to migrate is not caused by want of food. 
And as it is manifested to the same extent in their native habitats as in 
the temporary regions, when parasites are absent as well as present, it 
cannot depend upon the presence of parasites. 
As we have heretofore stated, Bowles assumes that the males are 
largely in excess of the females ; that their ardor is excessive, while the 
females, needing a larger supply of food, are always intent on feeding. 
That when the sun has dried off the dew in the morning, the females, to 
