206 MIGRATION 



during the afternoon and evening, but do not devour all the food be- 

 fore leaving an area, their departure being a response to stimuli other 

 than hunger (cf. Lean, 1931; Uvarov, 1931; Faure, 1932, 1935). 



Bodenheimer (1929) has made a careful study of the nymphs of 

 Schistocerca gregaria Forsk. with results similar to those of Parker. 

 He found that most of the nymphs emerge between 7 and 10 a.m. and 

 do not wander much during the first day, but remain near the egg site, 

 sunning themselves on stones, and some aggregating into bands. The 

 wandering nymphs of stages 1 to 5 differ little in life habits. Early 

 in the morning with temperatures below 20° C, nearly all rest on 

 plants. Later on, at 20° C. to 26° C, the nymphs assemble in aggre- 

 gations in warm places in the sun, turning their broad sides to the 

 sun and ceasing their wandering. At 27° C. and above up to 39° C, 

 migration takes place, the direction being determined largely by the 

 wind. With the rise of the soil temperature to 40° C, usually from 

 noon to 2 p.m., the insects arrange themselves wdth the long axis of 

 the body parallel to the sun's rays, which gives a minimum body 

 exposure. Some climb plants later in the afternoon, but as the tem- 

 perature falls, the insects assemble in dense swarms and arrange their 

 bodies to give maximum exposure to the sun; after sunset the majority 

 gather on plants, while a few remain on the ground, and during excep- 

 tionally warm nights they may migrate (cf. Filipjev, 1928, 1929, a, b). 



H. B. Johnson (1926) was the first who succeeded in actually 

 proving the transition between the migratory type, Schistocerca gre- 

 garia phasis gregaria Forsk., and the solitary type, S. gregaria phasis 

 fiaviventris Burm., in the Sudan. He discovered that the solitary 

 phase is the most numerous among the non-migratory locusts and that 

 the gregarious phase is typical of locust swarms. Bodenheimer (1929) 

 found that in Transjordania the hoppers, wdth a few exceptions, be- 

 longed to the migratory phase. He further states that the physiologi- 

 cal conditions that cause the accretion of migrant swarms and compel 

 the insects to remain in them are not clear. He quotes Fraenkel's 

 summary, however, as follows: "The social life of the locust may be 

 based on two fundamental instincts: (1) the aggregation instinct and 

 (2) the imitation instinct. The insects live, migrate, sun themselves, 

 and eat — always in groups. Apart from this, one animal imitates the 

 actions of its neighbor. It is due to the fact that the insects are 

 always together and each insect directs its body in accordance with 

 that of its neighbor, that mass migration takes place. H different 

 swarms meet, they immediately unite and migrate henceforth together, 

 for the imitative instinct impels the first to follow the direction of 

 movement of the second. It has not yet been determined as to how 



