GRASSHOPPERS 311 



spretus may possibly be traced to changes in its former breeding 

 grounds on the slopes of the Rocky Mountains from whence 

 swarms invaded the American plains. It is generally maintained 

 that the spread of agriculture has altered these breeding grounds 

 to an extent which has led to this loss of capacity to develop 

 into the gregarious phase. The form mexicanus, however, still 

 occasions severe damage in Canada and the United States. During 

 outbreaks it shows a definite tendency to form loose swarms and 

 to become semi-migratory or gregarious. The conditions which 

 might favour this kind of behaviour to an extent that would 

 lead to the development of a true migratory phase are unknown. 

 It is, however, noteworthy that specimens collected in the field 

 under outbreak conditions showed a strong tendency to vary in 

 the direction of spretus as regards wing length and in size. It 

 would seem, therefore, that the species mexicanus occupies an 

 intermediate position between the true solitary grasshoppers and 

 migratory locusts in that it becomes at times exceedingly abundant 

 and develops incipient swarms. 



Phase Development in Grasshoppers. It has been generally 

 believed that the existence of phases is a peculiarity belonging to 

 locusts and one not betrayed among non-migratory Acrididae, or 

 ordinary grasshoppers. Recently evidence has been brought 

 forward by Rubtzov (1935) showing that an essentially similar 

 phenomenon occurs in grasshoppers, but that the amplitude of 

 such phase differences is less pronounced than in locusts. Rubtzov's 

 important observations were carried out in Siberia, where he was 

 impressed by the colour variations found among individuals of 

 the same species of grasshoppers. Since some of the species in 

 which the phenomenon has been found occur often commonly in 

 Britain, a new field for study is opened up for workers in this 

 country. Certain species of grasshoppers were found by Rubtzov 

 to differ in coloration more or less in accordance with the 

 population density per unit area of territory. Individuals 

 collected from relatively dense associations, with up to 300 

 grasshoppers per square metre, were notably dark coloured, 

 larger, and had longer wings than those distributed in the 

 proportion of one, or fewer, over the same unit area. These 



