108 REPORT UNITED STATES ENTOMOLOGICAL COMMISSION. 
additional impulse. I think it more than probable th;tt the (-fleet of the 
dry and rarefied condition of the air upon them is such that when they 
arrive at the perfect state and the season of their amours arrives the 
migratory impulse is increased by irritation of some kind which causes 
an uneasy, restless feeling. Although I do not believe their migrations 
are for the purpose of carrying on their amours, yet I think it highly 
probable that at this stage of their lives the migratory impulse is strong- 
est, and that the excited condition of all the internal organs is probably 
one cause of its being strongest at this time. But that the migratory 
impulse does not depend upon the perfected condition of the insects is 
shown by the strong disposition of the young to migrate. Hence, the 
cause which operates to produce this effect acts upon the larva} and 
pupae as well as the perfect insects. 
There are souk; things connected with the movements of the Locusts 
which appear to indicate something more than an impulse simply to fly, 
which is the weak point of the theory if limited to irritating or uneasy 
sensations as the only cause of migration ; one is the evident disposition 
often manifested by the locusts to go in a given direction. This was 
.shown in our first report and need not be repeated here. 
That hunger will cause them to move from place to place in search of 
food is certainly true, but it would be a violent presumption to say that 
a swarm in Montana starts from there on a voyage to Nebraska iu 
search of food, or from Southern Russia to Germany. Having the mi- 
gratory instinct already implanted in them, and with it the adaptation 
to long flights, when once they start in search of food it is not incon- 
sistent with reason to suppose they continue on their course as driven 
by the wind until they find food or are exhausted. So far we can con- 
nect cause and effect, but we must confess that there are still facts 
•connected with their movements unexplained. For example, as will here- 
after be shown in the chapter on meteorological influences, those bred in 
the temporary regions in their attempts to return to their native breed- 
ing-grounds are not governed by the prevailing course of the winds, for 
if this were so some would go intone direction and some in another ; 
whereas those bred in Nebraska and southward exhibit a uniform dis- 
position to go northward or northwest even when the prevailing course 
of the wind is adverse. 
