GENERAL CAUSES OF MIGRATION. 
105 
avoid the importunities of the inqles, after repeated short nights, at 
length rise up and float away with the wind. 
Our only comment upon this eccentric writer's view is, that a careful 
count of a large number of specimens of G. spretus in different collections 
shows no marked difference in the number of one sex as compared with 
the other. In our First Report this subject was alluded to' 107 , and we see 
no reason to modify what is there stated. 
Keferstein remarks 208 that — 
The same causes which impel the Gryllus italicus to take the field, move also the 
Gryllus migrator ius to his wanderings, namely, want of food; the instinct of breeding, 
which the female, well knowing that thereby her life-aim will be accomplished, and 
she must then die, believes will be accomplished through the migration; and, finally, 
the instinct to seek out a suitable place of abode for their progeny. So then the flight 
of all species of locusts may be said to depend upon similar causes at bottom : neces- 
sity, love, and instinct drive them thereto. 
That the desire to seek places to deposit their eggs in localities which 
are best adapted to the young, influences locusts as well as other insects, 
cannot be doubted : and that this is one cause of migration is more thaa 
probable. But this will not account for the fact that they are essentially 
migratory even in their native habitats. Nor is there any reason, so far 
as we know, why it should operate in one species of Acrididce more than 
another when placed under the same circumstances. (Edipoda Carolina 
is found throughout the area occupied by C. spretus, and has ample wings 
to assist it in flight, but it is never found migrating in any true sense ; 
yet the maternal solicitude for the welfare of the young is doubtless as 
stroug iu the one species as the other. Neither of the reasons given, 
therefore, appears to be satisfactory. 
Darwin's opinion, as already quoted, appears to be, that excessive 
heat causes a kind oi* irritation or uneasy feeling in the locusts, which 
makes them restless and desirous of seeking some place where they can 
be sheltered from the rays of the sun. That some kind of irritation caused 
by excessive heat and dryness may render them restless and uneasy, is 
not only possible but probable ; but that this causes them to fly in search 
of the shelter of hills and forests is very questionable, especially when 
we take into consideration, as heretofore suggested, the well-known fact 
that, as an almost universal rule, they avoid forests and forest-clad areas. 
The only certain fact, then, that we have to start with in our investiga- 
tion as to the origin of the migratory impulse is, that it is in some way 
connected with a more than ordinarily dry and rarefied condition of the 
atmosphere. If we suppose this impulse or instinct to be once formed 
by such climatic conditions, we can then easily explain the flights in the 
lower plains of the temporary region where the atmosphere is more dense 
and humid. These conditions, combined with unusual heat, may pro- 
duce a kind of irritation in the air-tubes, and this may cause them to 
arise in search of a more rarefied condition of the air ; the moving breeze 
is found to favor respiration and tends to allay the irritation or uneasy 
807 Page 250. 
xwLoc. cit. 
