31 6 ANIMAL AGGREGATIONS 



other aphids and the possibihty of aphid secretions being transferred 

 from one to another through the plant. Merely stating these alter- 

 native hypotheses helps to emphasize the probability that changed 

 or decreased nutrition is the dominating factor in the situation. 

 This recital of references to observations by a number of workers 

 in widely separated regions, and usually upon distinct species, indi- 

 cates that the effect of crowding upon wing production must be a 

 widespread phenomenon among aphids. The racial importance of 

 this phenomenon is apparent when one remembers the added 

 migratory power thus conferred upon members of a crowded colony 

 living upon a host plant which may soon become exhausted from 

 the feeding activities of its aphid population. 



CROWDING AND THE PHASE THEORY OF LOCUSTS 



In the following discussion it will be convenient to follow Uvarov 

 (1928) in applying the name "locust" to gregarious members of 

 the short-horned grasshoppers, family Acrididae, which migrate in 

 swarms, while "grasshoppers," in a restricted sense, will be used in 

 speaking of the non-gregarious, non-migrating members of the same 

 group. The gregarious collections of adult locusts will be spoken 

 of as "swarms," and the similar collections of immature hoppers 

 (nymphs) will be called "bands." 



In order to have clearly in mind the morphological relations of 

 locusts and grasshoppers, it will be necessary to pass hurriedly in 

 review some of the available knowledge concerning the behavior of 

 these animals. The egg pods of the gregarious locusts are deposited 

 close together so that when the young hatch and emerge from the 

 ground they are immediately in close contact with each other. After 

 their intermediate molt these recently emerged hoppers soon form 

 primary bands, due largely to the reaction of the young animals to 

 light and heat. On warm sunshiny days these gather in exposed 

 sunny places; on cool wintry days the bands collect in sheltered 

 spots (La Baume, 1918). In addition to these place aggregations 

 there is said to be a distinct gregarious tendency which has not yet 

 been analyzed to see whether it is a behavior unit or whether it may 

 be split into more elementary reaction complexes. 



The alternation of behavior, night and day, appears to be related 



