154 B. p. UVAROV. 



which is easily accessible and mostly cultivated or, anyhow, populated. I believe, 

 therefore, that permanent breeding grounds of migratorioides are to be looked for 

 somewhere deep in the impenetrable jungles, overgrown with high grasses, reeds, 

 and such-like vegetation ; but even if I am mistaken in this supposition, it would 

 not affect my theory, which is based on the indubitable fact that the permanent 

 breeding of migratorioides is possible only in localities with certain natural conditions, 

 whatever those conditions may actually be. 



Another well-known fact is that the development of migratorioides in its breeding 

 grounds does not go on always at the same rate, but that it is subject to a periodical 

 rise and fall, though the exact cause of the increase of locusts is entirely unknown. 

 When the increase is at its height, large swarms are formed, and their emigration 

 follows. Such emigrated swarms settle down and oviposit whenever they are 

 compelled to do so by purely physiological causes, and their progeny undergoes a 

 transformation into the solitary-living phase — danica. The very plastic, easily 

 adaptable, and in all respects more progressive danica must play an important part 

 in the extension of the range of the species, gradually but steadily populating new 

 regions. Being a product of a mutation arising partly from some unknown internal 

 cause and partly from outer (probably climatic) influences, danica is naturally subject 

 to sudden displays of atavism, which results in the transformation into the ancestral 

 phase migratorioides. We do not know yet whether this phenomenon can occur 

 spontaneously as a result of some internal physiological factor, but there is no doubt 

 that it is much favoured and often probably caused by the oviposition of danica 

 taking place under conditions like those of the permanent breeding grounds of 

 migratorioides. The gregariousness of the migratorioides phase is, of course, one of 

 the causes of a rapid increase in the number of individuals and swarms, and soon — 

 in the course of a few generations — the size of the swarms reaches the maximal 

 point, which is followed by emigration. In this way the dispersion of the species 

 goes on alternately by the gradual spreading of the danica phase and by the periodical 

 extensive emigrations of migratorioides. As a result, the species is now distributed 

 all over the Eastern hemisphere ; but, as we know, the distribution of migratorioides 

 is limited to tropical regions only, while danica goes over to the Palaearctic region 

 as well, where the swarming phase of the species is represented by migratoria. This 

 latter fact might be satisfactorily explained by the impossibility of finding in the 

 Palaearctic region the natural conditions exactly like those of the tropical breeding 

 grounds of migratorioides, chiefly in regard to a combination of heat and dampness. 

 The above-described (p. 143) reed-beds of Phragmites in the south-eastern part of 

 the Palaearctic region represent in all respects the nearest possible approach to 

 tropical conditions. This statement is strongly supported by the fact that the 

 fauna of these reed-beds includes two more Acridians of an undoubtedly tropical 

 origin ; these are Gelastorrhimis sagitta, Uvar., and Oxya tiiranica, Uvar., both 

 described* from the valley of the Amu-Darya, in Transcaspia, and the former found 

 also on the River Kura, in Transcaucasia. Though very peculiar, and in the summer 

 recalling the tropics, the climatic conditions of these reed-beds are, of course, not 

 tropical, and their effect on the progeny of danica breeding there is not the same 

 as in the tropical breeding grounds of migratorioides : the reverse transformation 

 of danica into a swarming phase does not reach the phase of migratorioides, but stops 

 half-way at the migratoria-phsise. This seeems to indicate that the transformation 

 is due primarily to the direct influence of external conditions, its extent being 

 proportional to changes in the latter, but only precise laboratory investigations can 

 help to clear up this complicated problem. It is interesting to recall here that 

 individuals of migratoria incidentally occur in tropical countries also (see p. 152), 

 and we may presume that their appearance is due to some abnormal conditions 

 of the development. 



* Horae Soc. Entom. Ross, xl. No. 3, 1912. 



