A REVISION OF THE GENUS LOCUSTA, L. 147 



therefore, that the cessation of flight is caused entirely by these two purely physiological 

 phenomena, and it is obvious that no possibility exists for swarms to look for 

 suitable new breeding grounds ; they merely settle down wherever they are compelled 

 to do so by their physiological condition, quite irrespective of the character of the 

 locality. This may occur accidentally near some suitable spot, and there is also 

 the probability that the swarm would find one during the irregular circular flights 

 which take place at the end of the emigration and precede pairing and oviposition ; 

 but the extent of these flights is rather limited and this probability is correspondingly 

 small. Theoretically it is far more probable that the cessation of emigration would 

 become necessary in a locality quite unlike the normal breeding grounds. 



A typical and very convincing example of this kind was observed in Stavropol 

 province in the autumn of 1912, when numerous large swarms of migrator ia 

 emigrating from the breeding area at the mouth of the river Terek invaded that 

 province. One or two of these swarms settled down in the lower portion of the 

 valley of the river Kuma, which is itself a breeding region of the same locust, but 

 was in that particular year free from the local swarms ; the Terek swarms con- 

 sequently found there most favourable conditions and oviposition took place in the 

 normal manner. Practically all the other invading swarms stopped their flight 

 in the steppe adjoining the middle course of the Kuma, some of them on the very 

 border of the valley. Now this part of the valley presents some very suitable breeding 

 grounds, which have often played an important part as the source of invasions in 

 Stavropol province. Several of the swarms visited these grounds more than once during 

 their circular flights, which are often supposed to serve the purpose of finding 

 suitable places for oviposition. Ultimately, however, only a small number of 

 scattered locusts oviposited there, while all the swarms deposited their egg-masses 

 in the dry steppe, where the conditions of soil and vegetation are entirely different 

 from those in normal breeding grounds. It is especially interesting to note that 

 some of the eggs were laid on a portion of the steppe sloping towards the valley of 

 the river, i.e., in the closest proximit}^ to the above-mentioned suitable area. 



This latter fact and a study of the general conditions under which oviposition 

 took place clearly show that nothing in the least like a conscious (or instinctive — 

 the exact word does not matter in this case) choice of suitable places by swarms can 

 be assumed. There is, however, one exception : when a swarm settles down for 

 oviposition, and the females, after several attempts to penetrate the soil, find, it too 

 hard, they become restless, take wing again, and after a few rounds settle down at 

 another spot. Thus we must conclude that oviposition takes place whenever the 

 majority of females are ready for it, and quite irrespective of the suitability of the 

 conditions for the next generation, provided that oviposition is physically possible. 



In conclusion, the theory that emigration has as its aim the finding of new 

 breeding places is also groundless, and there is at present no possibility of explaining 

 the emigration by any causes except physiological ones : the development of the 

 air-sacs compels the insects to fly, and this impulse is strengthened by their gregarious- 

 ness, that is by some kind of tropism which makes each individual keep close to its 

 fellows and follow their movements. Later on we shall see what is the biological 

 meaning of the emigration. 



Such is, briefly, the life-cycle of migraioria. The biology of the larvae and adults 

 of danica is only very insufficiently known,* but what is known shows that their 

 behaviour is entirely "different from that of niigratoria. The chief biological feature 

 of the latter in both larval and adult stages — gregariousness — is quite absent in 

 danica. This is especially striking in the larvae ; if a wandering swarm of migrator ia 



* This is directly due to the fact that most entomologists have regarded danica as distinct 

 from niigratoria and as being an entirely harmless species, so that the study of its habits has been 

 neglected. 



