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UvAROv (B. p.). A Revision of the Genus Locusta, L. (= Pachytyhis, 

 Fieb.), with a new Theory as to the Periodicity and Migrations of 

 Locusts. — Bull. Ent. Res., London, xii, pt. 2, September 1921, 

 pp. 135-163, 8 figs. 



Not less than 16 species have been attributed to the genus Locusta, 

 L., by different authors. This number had already been reduced by 

 earlier revisers, but Kirby in his Catalogue of Orthoptera still mentioned 

 seven distinct species. The author's conclusion — based on the morpho- 

 logical analysis of very extensive series of specimens, as well as on 

 field observations conducted for several years, and on breeding 

 experiments — is that the species of Locusta present a far greater 

 variability than might have been anticipated, and that only two 

 species can be distinguished, namely, L. migratoria, L., and L. 

 pardalina, Wlk. ; the latter, however, differs from L. migratoria in 

 so many important characters that a new genus, Locustana, is here 

 erected for it. 



The inter-relations of the three more or less constant forms of 

 L. migratoria are fully discussed, and the conclusion is drawn 'that these 

 three forms — migratoria L., migratorioides, Rch. & Frm., and danica, 

 L. — cannot be separated specifically, and that they represent taxonomic 

 units of lower grade than a species ; they are, however, quite distinct 

 from each other, though connected by transitional forms and even 

 able to undergo a transformation (during two or more subsequent 

 generations) into each other. These forms the author proposes to 

 call " phases " of the species. The oldest, ancestral phase of the 

 species is considered to be migratorioides, which is far more constant 

 in its morphological and colour characters than the more plastic 

 migratoria and still more variable danica. It is distributed all 

 over the tropics and subtropics of the Eastern Hemisphere (to 

 which the whole genus Locusta is peculiar, since all records of the 

 occurrence of it in America are incorrect) ; its permanent breeding 

 regions have not yet, however, been located. 



The best conditions for the development of this form seem to be 

 present in tropical countries with a rather damp and hot climate ; 

 the sites of breeding grounds are certainly not forests, but most 

 probably impenetrable jungles overgrown with high grasses, reeds 

 and similar vegetation. The development of migratorioides in its 

 breeding grounds is subject to a periodical rise and fall, though the 

 exact cause of this is as yet unknown. When the increase is at its 

 height, large swarms are formed, and their emigration from the breeding 

 grounds follows. Such emigrating swarms settle down and oviposit 

 whenever they are compelled to do so. Physiological causes, such as 

 reduction of the air-sacs, exhaustion of the fat-body and development 

 of the reproductive organs are the sole factors that bring this about. 

 The resulting progeny undergoes a transformation into the solitary 

 danica phase. 



The danica phase, which is very plastic, easily adaptable, and 

 cryptically coloured in all stages of its development, plays 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 external (probably 

 cHmatic) influences, it is subject to sudden displays of atavism, 

 resulting in the reappearance of the ancestral phase, migratorioides. 

 This phenomenon is favoured and probably often caused by the 

 oviposition of danica taking place under conditions resembling those 



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