146 B. p. UVAROV. 



Still more unacceptable becomes the theory of emigration being caused by lack 

 of food, if we turn our attention to certain physiological changes which locusts 

 undergo during the period of emigration. WTien dissecting individuals taken from 

 emigrating swarms, it will be found that by far the greater portion of the inner 

 cavity of the body is occupied by air-sacs, described long ago by American ento- 

 mologists in the Rocky Mountain locust, and occurring doubtless in all other 

 migrating species of locusts. These air-sacs are only temporary organs, reaching 

 their highest development at the period of emigration and disappearing towards the 

 end of that period, when the developing reproductive organs' take their place. 

 During the emigration, however, the air-sacs are enormously large and all the other 

 internal organs are much compressed, including the stomach, thus rendering the 

 insect almost incapable of taking food, at any rate in large quantities. This 

 assumption, based upon anatomical facts, is supported also by field observations ; 

 for, in fact, the emigrating swarms, when they stop their flight, do not feed much, 

 though incidentally they may cause great damage by merely cutting the stems 

 of cultivated plants. 



Further anatomical researches reveal also the fact that the fat-body is more 

 developed in insects just before and at the beginning of emigration, and is almost 

 exhausted towards the end of it ; probably locusts during this period live essentially 

 on the food reserves in the fat-body, being unable to take much vegetable nourish- 

 ment and consequently scarcity or even lack of food has nothing to do with the 

 emigration. 



A Russian entomologist, K. N. Rossikov, called attention to another possible 

 explanation of the emigration of migraioria from its breeding grounds. He believed 

 it to be an immediate result of the activity of the parasites of the adult locusts, 

 i.e., Sarcophagid larvae and red mites ; he believed that the parasitised individuals 

 become restless and try to get rid of their parasites by flight. If this were so, the 

 result would be that only parasitised individuals would emigrate and all the non- 

 parasitised would remain behind in the breeding grounds, though actually just the 

 opposite is observed. This theory, therefore, is as groundless as the previous one. 



One more theory is that emigration might be regarded as a tendency of the 

 species to avoid overpopulation of a breeding region and to find new suitable breeding 

 grounds. As for the possibility of overpopulation of breeding regions of migraioria, 

 this idea is simply absurd, since these regions are vast enough to harbour many hundred 

 times more locust swarms than there are in years of maximal development. 



To investigate the presumption that swarms are emigrating to look for new 

 breeding grounds we must see what is the fate of swarms after they have left their 

 permanent breeding regions. 



As far as is known at present, a swarm of emigrating locusts usually covers a 

 very long distance at one flight ; if sometimes it settles down on its way (and this 

 is often caused by imfavourable conditions of weather), it soon resumes its flight 

 again. I will not discuss here the question of the direction of the flight and its 

 probable causes, since but very little is known about it. One fact, however, is firmly 

 established and is of great importance for our immediate purpose : it is that in the 

 majority of cases the swarms maintain throughout the same more or less defined 

 direction which they assumed when starting ; of course, a strong wind or other 

 incidental circumstances may to a certain extent alter this dii"ection. 



This straight flight, aimless and causeless as it seems, does not last long, though 

 a swarm may cover during it very long distances, the velocity of flight being far 

 greater than might be expected. 



Sooner or later, the regularity of the flight seems to be lost ; swarms begin now 

 to settle down, then take wing again and circle about ; they begin also to eat more, 

 since their air-sacs have grown smaller and the fat-body is also exhausted. I believe. 



