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clcs. ill the leaf-hoppers on the other hand there seems httle 

 (htTerence in this respect, proba'bly due to the fact that tiie 

 thoracic muscles are already somewhat feeble and little adapted 

 for strong flight. 



In polymorphism there are two well marked forms, the ex- 

 tremes, that is, of macro ptcr ism and hracJiyptcrisui, as well as 

 numerous intcnncdiatc forms sometimes. 



In older times, it was supposed that the brachyplerous (or 

 ai)ter()us) forms were sexually immature, and later, When mac- 

 ropterous and apterous forms were observed in copulation, it 

 was supposed that they were nymphs capable of procreation. 

 These apterous or brac'hypterous forms 'have been reared from 

 ordinary nymphs and are completelv mature as regards their 

 sexual apparatus. 



The genus PerkinsieUa affords a good example. There are 4 

 known species, closel)^ allied but abundantly distinct. Two oc- 

 cur in Australia (one also in Hawaii) one in Viti, one in Java; 

 the first three have been abundantly investigated by Koebele 

 and Perkins, the fourth by Breddin. 



(i) saccharicida Kir'kaldy: the males always macropterous, 

 the female is di- (or more or less poly-) morp'hic. 



(2) grainiiiicida Kirkaldy : as far as known the male is always 

 macropterous, the female brachypterous. 



(3) 7'ificiisis Kir'kaldy: both sexes are dimorphic pterygially. 



(4) I'astatri.v Breddin: similar to I'ificnsis. 



It has been supposed by Flor and Sahlberg that this poly- 

 morphism is caused by climatic influence. These authors be- 

 lieve that this inability to develop properly the tegmina and 

 wings, does not exist em'bryonically in the egg, but depends on 

 accidental conditions, such as the nourishment of the nymphs, 

 and above all on the state of the climate during fheir develop- 

 ment. Salilberg adds that the 'feeble development of the organs 

 of flight is altogether due to the greater develooment of the 

 other organs of motion, i. e., the legs. Renter believes, how- 

 ever, that the increa'se in number of polymorphic species as one 

 proceeds northwards in Europe depends probablv on many 

 other causes, independent of climatic influence. Thus only 

 eig'ht species of Auchenorhynchi live in Lapland on trees or 

 'bushes, and these species are always nionomorphic. 



Renter makes the following observations: 



(\) Dimorphic forms are found even in the tropics. 



(2) S'hort-winged individuals are observed most fiecjuently 

 among insects whose legs are well developed and constructed 



