314 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1952 



recent years (Hebard, 1917, p. 271). A more intriguing puzzle, one 

 with thoroughly practical aspects, concerns the almost complete dis- 

 appearance of M. spretus after the last big flights of the 1800's. Oc- 

 casional specimens commonly considered to be abnormally long- 

 winged representatives of the lesser migratory grasshopper are found 

 from year to year, and during certain years moderately important 

 flights of that species take place, but in spite of its importance as a 

 pest there is no recurrence of the tremendous, devastating flights of 

 the last century. It is now believed, with reasonable certainty, that 

 only one species of grasshopper is involved in this particular problem ; 

 to it the earliest name (mexicanus) applies. The species is capable 

 of undergoing changes in its behavior and appearance as a result of 

 the ecological conditions of its environment. Thus, spretus is a syn- 

 onymous name, and the common name Rocky Mountain grasshopper 

 applies primarily to the optimum migratory form of the species. 



THE PHASES OF MIGRATORY GRASSHOPPERS 



The realization that a few species of grasshoppers, apparently in- 

 cluding the majority of the major migratory grasshoppers of the 

 world, 3 have this ability to modify their color, wing length, and tend- 

 ency to migrate in swarms is one of the basic entomological discoveries 

 of our generation. It is extremely important because destructive 

 migratory species formerly would seemingly disappear after a period 

 of abundance, and frequently in their place only small numbers of 

 scattered and relatively unimportant grasshoppers could be found. 

 Later, the migratory pest form would reappear in an unexplained 

 manner. Credit for first suggesting that this transformation occurs, 

 and for reporting the first observations that demonstrated it in the 

 field, belongs to Dr. B. P. Uvarov, now director of the Anti-Locust 

 Research Center in London. This idea, which became known as the 

 Phase Theory, was proposed in 1921, and by various entomologists, 

 especially those working in Africa, it has been advanced from a theory 

 to the realm of fact. It has led to a much more realistic approach 

 to the whole problem of migratory grasshoppers, 4 and places the 



3 In the United States and Canada there are some 600 species of grasshoppers, not 

 including katydids, crickets, and other relatives. Of the 600, several dozen have destruc- 

 tive habits in relation to cultivated crops or range land. Of this number, only four are 

 well known for their migratory flights. About a dozen highly important migratory grass- 

 hoppers infest foreign countries, among several thousand grasshopper species that have 

 been described and named. 



4 The term "locust" as usually used means a grasshopper that is outstandingly migra- 

 tory in its behavior. Foreign entomologists usually use "locust" in that sense and 

 apply "grasshopper" only to the species that lack conspicuous gregarious and migratory 

 habits. There is no clear line of distinction between the two, however, since both types 

 of species belong to the same group and certain species are sometimes solitary and at 

 other times migratory. Furthermore, some writers on the classification of grasshoppers 

 refer in a general way to whole subfamilies of grasshoppers as "locusts," irrespective of 



