36 THE SOCIAL LIFE OF ANIMALS 



the Sahara borderlands, in southern Russia, in 

 South Africa and on the Malay Peninsula in ter- 

 rorizing numbers (Figure 1). They once did so on the 

 Great Plains of the United States, leaving a lively 

 memory of destruction that is still roused by the 

 smaller migrations that may occur there any summer 



^''' ^ ■.■:,:■:. . ■■ . -•■•■■■•• \ 



Fig. 1. A band of grasshopper nymphs on the march. 

 (From Uvarov, by permission of the Imperial Bureau of 

 Entomology.) 



in spite of active control measures. I myself have 

 seen the so-called Mormon cricket advancing from 

 the relatively barren mountain pastures of Utah 

 into the green fields in numbers which were not 

 halted by the hawks, turkeys and snakes attendant 

 on the swarm and feeding greedily; or the active 

 assaults of men and children warned out to protect 

 the cultivated lands. Migrating army worms and 

 chinch bugs present equally impressive aggregations. 

 The emergence of Mexican free-tailed bats from 

 the Carlsbad cave of an August evening has been 

 described as a black cloud pouring out in such den- 

 sity as to be visible two miles away. (19) Such bats 



