The physiological requirements of the Desert Locust do not suggest a high degree 

 of adaptation to what is usually understood by desert conditions. The female locust 

 lays eggs preferably in sand, but not in dry sand. Sand which is dry on the surface 

 but m9ist underneath, is suitable. Other loose soils are also acceptable, provided 

 they are moist. In the absence of moisture, eggs may be laid on the surface of the soil 

 where they perish. A developing egg of the Desert Locust has very high humidity re- 

 quirements, since it needs to absorb more than its own weight of water for successful 

 development (Shulov, 1952). Therefore, the soil round the egg must preserve suffi- 

 cient free moisture for some 12- 15 days of the incubation period. 



When young locusts (hoppers) hatch, they must have sufficient green food, usually 

 tender annual plants which rapidly spring up in sandy desert areas after a shower of 

 rain. Excessive heat and dryness during the hopper development, which takes 5-6 

 weeks, has been known to cause wholesale mortality of hoppers. 



Adult Desert Locust are known to be able to survive for many months in condi- 

 tions of dryness, but for their sexual maturation they require either succulent green 

 food or high air humidity (Hamilton, 1950; Norris, 1952); in this respect, desert con- 

 ditions are definitely unfavourable for reproduction. 



With regard to activity, it has been thought before that adult locusts, particularly 

 in swarms, would be most active in intensely dry and hot conditions and this would 

 cause them to migrate to more favourable habitats, but recent observations (Waloff, 

 1952) tend to dispel this idea, since flight activity of swarms appears to be more per- 

 sistent at higher air humidities than at the lower. 



It would appear, on the whole, that the Desert Locust is far from well adapted to 

 general desert conditions, and the question arises how can the species not merely 

 survive, but be able, from time to time, to multiply in fantastic numbers. 



The answer is to be sought in the fact that the widespread conception of a desert 

 is too generalised. It covers a great variety of landscapes, which provide desert ani- 

 mals with a wide range of habitats, some of them offering very favourable conditions 

 of life. In addition to this variety of conditions in space, there is a great seasonable 

 variation in all life conditions : a truly desert, lifeless area becomes covered by lush 

 annual vegetation almost immediately after a shower of rain. 



The existence of such favourable ecological islands is the essential condition for 

 the existence of the Desert Locust. Since, however, many of such islands are only 

 ephemeral, they are clearly unable to support a stationary locust population. On the 

 other hand, an insect which is capable of moving from one favourable area to another, 

 has an excellent chance of survival, and the continuous existence of S. gregaria in the 

 desert regions is closely bound up with its migratory or, rather, nomadic habit. 



Most biologists would be content to accept the beneficial value of migration as a 

 sufficient explanation of the migratory pattern, but it is possible now to offer a some- 

 what deeper analysis of this phenomenon. 



Studies of Desert Locust migrations in relation to weather factors (Waloff & Rainey, 

 1951; Waloff, 1952) have shown that swarm displacements are closely linked up with 

 weather dynamics, and Rainey, (1951) put forward a well - documented hypothesis that 



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