32 REPORT UNITED STATES ENTOMOLOGICAL COMMISSION. 
characteristics or differences of sufficient importance to mark the groups 
of the family. If we examine the species of the different restricted gen- 
era containing the more noted locusts, as Pachytylux miyratorius, Acrid- 
ium pcrcyrinum, Caloptenus italicus, and Caloptenus spretus, not only do 
we find the same thing true in reference to them, but also that in some 
cases the most closely allied present the strange fact that one is truly 
migratory while the other is not ; as, for example, Caloptenus spretus and 
Caloptenus femur -rubrum of our own country, and Acridium pereyrinum 
of the eastern continent, which is scarcely distinguished from Acridium 
americanum of North America. It is therefore evident thai we must 
look to something else than such anatomical differences as distinguish 
genera for the cause of the migratory disposition. Before entering upon 
the discussion of this peculiarity in habits, we will first discuss the facts, 
so far as known, in regard to their migrations, with reference chiefly to 
the Rocky Mountain locust of our own country, but referring to other 
species for illustration and to correct what we conceive to be erroneous 
opinions. 
In our investigations of the migrations of the Rocky Mountain locust 
we have found it necessary to direct our attention to the following points 
as the ones of chief importance, and as forming the principal divisions 
of the subject : 
1. Native breeding ground or point of departure, its position, bound- 
aries, physical features, climate, &c. 
2. Habits or characteristics of the species within this area of perma- 
nent distribution, so far as these relate to their movements ; especially the 
question as to whether they are essentially nomadic or sedentary. 
3. Their flights. 
4. Climatic conditions. 
5. Causes of migration. 
6. The effect or consequences of these migrations. 
The present chapter will be limited strictly to a consideration of the 
"philosophy of migrations," the last and all important topic — "conse- 
quences" — to which the attention of the Commission has been largely 
devoted, being treated of in other chapters. 
I. — A BRIEF ACCOUNT OF LOCUST MOVEMENTS IN OTHER COUNTRIES. 
As we shall have frequent occasion to refer to the migrations of locusts 
in other countries as well as our own, we will give a brief general review 
of locust movements in various parts of the world, limiting ourselves 
strictly to such facts as have a direct bearing upon the question under 
discussion. For the facts mentioned in this part of the chapter we are 
largely indebted to the following writers, but have endeavored, by a la- 
borious consultation of the authorities quoted, so far as to be found in 
the Congressional and other libraries of our country, to verify these 
quotations : 
K. Ritter. — "Die Heuschreckenplage der Lander der Alten Welt,. 
