A REVISION OF THE GENUS LOCUSTA, L. 155 



Little is known yet as to what happens in the breeding grounds after the 

 emigration,* except that the number of locusts drops suddenly to a minimum. 

 I presume that scattered swarms of the gregarious phase, as well as the progeny 

 of individuals of the solitary phase, cause the gradual increase in the number of 

 swarming individuals, and after a few years a new emigration occurs. 



Thus, the periodicity of locust invasions is caused entirely by the wonderful 

 phenomenon of the transformation of a swarming locust into a solitary, harmless 

 grasshopper. Of course, the outline here sketched is necessarily rough, and the 

 actual proceedings are far more complicated, but the theory seems to me to be the 

 best possible in the circumstances. 



The biological result of these phenomena is that the maintenance and dispersion 

 of the species is ensured in all circumstances : the swarming phases enable the 

 species to extend at one stroke its area of distribution to distant regions, and its 

 dispersion to the remotest islands is undoubtedly due to emigrating swarms ; on 

 the other hand, the well protected and easily adaptable solitary phase secures a strong 

 footing in the countries thus reached, and under favourable conditions gives rise to 

 new emigrants ; the results achieved show that such an arrangement has been 

 extremely useful to the species. Even the most radical changes in the natural 

 conditions of the permanent breeding regions would result not in the extermination 

 of the species, but only in its transformation into the more adaptable danica phase. 



An example of that kind occurred in Southern Russia. Though the now existing 

 permanent breeding regions are restricted, as I. have described above (p. 143), 

 to the valleys of the rivers discharging into the Caspian and Aral Seas and Lake 

 Balkhash, the deltas of rivers emptying into the Black Sea {i..e., Kuban, Don, Dnieper, 

 Danube, etc.) also harboured not very long ago — up to the end of the eighties of the 

 last century — ^some permanent breeding grounds of migratoria. At present, however, 

 only the lower valley of the Danube is still a breeding region, while the valleys of 

 the other rivers of the basin of the Black Sea no longer serve that purpose. This is 

 easily explained by the fact that the valleys of the Don, Kuban and Dnieper were 

 during the end of the last century more or less cultivated or, at any rate, their 

 natural conditions were entirely changed by the persistent grazing of herds of cattle. 

 As a direct result of this the possibility of the transformation of the solitary phase 

 into the swarming one exists there no longer, and though the transformation takes 

 place incidentally, single specimens of migratoria being not uncommon, their numbers 

 do not increase, nor are swarms ever found. 



The theory of phases suggests the theoretical possibility of the control of migratoria 

 by some measures directed not against the insect itself, but against certain natural 

 conditions existing in breeding regions which are the direct cause of the development 

 of the swarming phase. The above-quoted example in South Russia shows that 

 even comparatively slight cultivation of breeding regions leads to the desired changes ; 

 but the conditions necessary for the breeding of the swarming phase have not been 

 exactly studied, nor are the direction and extent of such changes known. The 

 first step, therefore, should be the most careful investigation of all existing, as well 

 as extinct, breeding regions, together with parallel breeding experiments under 

 laboratory conditions ; on the basis of results thus gained a system of theoretically 

 useful and practically possible measures for the conversion of breeding regions may 

 be outlined. 



IV. LOCUSTANA PARDALINA, WALK., AND ITS PHASES. 



My personal knowledge of this locust is limited to the study of preserved specimens, 

 especially of a large series sent to the Imperial Bureau of Entomology by Mr. J. C. Faure, 



* The direct cause of this ignorance is that injurious insects, and locusts especially, are 

 studied only in the years of maximum development, and nobody cares about them in the minimum 

 years, when the clue to the whole locust problem is most likely to be found. 



(3442) M 2 



