3i8 ANIMAL AGGREGATIONS 



the movement of the band determined by chance or by environmen- 

 tal factors. La Baume (1918) thinks that the bands of the Moroccan 

 locust move downhill because of a positive geotropism. Most of 

 the obvious environmental factors, such as direction of the sun's 

 rays, direction of wind, location of lush vegetation, do not seem to 

 be definitely related to the direction of movement. 



The appearance of one of these bands on the march is shown in 

 the accompanying figure from Uvarov, from which it may be seen 

 that the line of movement tends to be broad and shallow and with 

 an irregular front (Fig. 31). When two such wandering bands meet, 

 they usually fuse and go off in the direction formerly taken by the 

 larger band. Such migrating bands recognize no obstacles other 



....^: 



Fig. 31. — A band of locust hoppers on the march. (From Uvarov 1928, by permis- 

 sion of the Imperial Bureau of Entomology.) 



than smooth vertical walls, and these merely cause a deflection. 

 Inequalities of the surface are filled by the bodies of the first comers, 

 and those following pass over the smoothed surface. Rivers are 

 crossed by swimming with the same hopping motions that carry 

 the insects along on land. The bands stop at noon if the heat be- 

 comes sufficient to produce heat torpor, and at night when cold 

 torpor sets in. 



We have already seen that the beginning of the wandering, as well 

 as other movements of these hoppers, is largely determined by the 

 temperature. We shall see later that, according to the phase theory 

 of Uvarov, living together in dense bands tends to cause an altera- 

 tion in the coloration, so that black pigment develops. Buxton 

 (1924) reported that the body temperatures of a black form of 

 Calliptamus coclesyriensis, a grasshopper of Palestine, was from 4° 

 to 5° C. warmer than buff individuals of the same species under the 

 same conditions. If Uvarov's theory is correct, it would appear 



