250 REPORT UNITED STATES ENTOMOLOGICAL COMMISSION. 



and have been considered in Chapter VII. It is, therefore, of the imme- 

 diate causes that we shall speak in this connection. Were we asked 

 to give any single explanation of the phenomenon we should answer, 

 excessive multiplication ; for this is evidently the immediate cause, and 

 the others here considered are mostly secondary or but consequences of 

 this one. Of these secondary exciting causes we would instance the 

 following as worthy of consideration : 



1. Hunger. — Whenever food is lacking, whether through the excessive 

 droughts that are not uncommon in the insects' permanent habitat, or 

 through such excessive multiplication of the species that all vegetation 

 is devoured before the life-course of the individual is completed j there 

 must needs be the strongest incentive to migrate. Such we find to be 

 the case, under like circumstances, with many animals normally non- 

 migratory. Hunger, then, may become an incitation to migration, even 

 where there is not excessive increase. 



2. The procreative instinct — From extensive observation we are con- 

 vinced that there is a natural tendency in the species to seek for fresh 

 breeding-grounds away from the location of birth; yet, even were there 

 no instinctive tendency of the kind, we may find a sufficient incentive for 

 movement from one place to another during the season of procreation^ 

 in the well known salacious habits and ardor of the males. Whenever 

 the insect is very abundant, the females are much annoyed and disturbed 

 during the act of oviposition, as several males will be constantly attend- 

 ing her ; and it is more than probable that she is often driven to take 

 wing in consequence. 



3. Increase of and annoyance from statural enemies. — No one who has 

 witnessed the excessive abundance in which some of these natural 

 enemies of the locust at times prevail, can for a moment doubt that they 

 often prove a valid cause of migration. The Tachina-flies, especially, 

 have been known to follow the locusts in dense clouds, and we have 

 seen them so thick in parts of the West that not a locust could rise from 

 the ground without being pursued by several; and there is no escape 

 from their pursuit until the persecuted victim gets high in the air. 



4. Instinctive impulse. — Though at first we were unwilling to allow any 

 instinctive guidance in the migrating movements, the more we study 

 the question the more we are inclined to consider as a factor in the 

 problem a certain instinctive prompting to that wliich is best for the 

 preservation of the species. The persistent movement in a given direc- 

 tion notwithstanding adverse winds, so often recorded in this report; 

 the fact that there is a well marked eastern limit line; and the return 

 migration from the Temporary region, are not, to the same extent, sus- 

 ceptible of any other explanation. Hunger, or excessive multiplication, 

 though we grant them to be important causes of the migration from the 

 Permanent region, generally have little to do with this retiirn migra- 

 tion ; because, as we have seen, the insects all leave, whether few or 

 many, and they pass over great stretches of luxuriant vegetation, both 



