MORPHOLOGICAL EFFECTS OF CROWDING 321 



given by nearby grasshoppers. In behavior there is no hard and 

 fast Hne that can be drawn between the two types, and in matters 

 of form and color it appears that they also intergrade to a consider- 

 able degree. 



This lengthy introduction to Uvarov's theory of phases of locusts 

 is needed in order to have a proper background to understand the 

 relations considered in that theory. To this, another known rela- 

 tionship should be added. After a period when non-gregarious 

 grasshoppers have been no more than the usual agricultural pest, 

 taking a relatively light toll of the available plants, a locust outbreak 

 may occur either suddenly or after building up for a year or so; 

 and this may be so serious as to present a great agricultural problem 

 for an entire district. Such an outbreak may disappear as suddenly 

 as it appeared, leaving behind only the normal population of grass- 

 hoppers. These outbreaks do not appear to have any definite pe- 

 riodicity. They are probably conditioned by a favorable combina- 

 tion of climatic and biotic factors. Weese (1924), working with 

 spiders and their parasites, found that the climatic conditions which 

 favored the development and survival of the hosts differed from 

 those which were most favorable for the development and low mor- 

 tality of their parasites. Uvarov is probably right in saying that 

 "neither climatic factors in themselves nor the activity of natural 

 enemies can be regarded as sufhcient for a satisfactory explanation 

 of the rapid increase in numbers of locusts in their breeding grounds 

 at the beginning of an outbreak"; but he probably underestimates 

 the possibihties of the two factors working together to supplement 

 and reinforce each other. 



In order to account for locust outbreaks and their sudden subsi- 

 dence, Uvarov has put forth his theory of locust phases (1921,1928). 

 The essence of this theory is: "Various species of gregarious locusts 

 cannot be considered absolutely stable in all their characters, either 

 morphological and (sic) biological; on the contrary, there are good 

 reasons for regarding each species as exceedingly plastic and liable 

 to fluctuations in all essential characters. These fluctuations have, 

 of course, certain Hmitations, but in some cases the bounds are so 



