334 Experitnental Zoology 



turely to other plants or by preventing the normal migration? 

 A few attempts that I have made in this direction show that the 

 experiment is not always easily carried out, at least in the forms 

 that I have used, since if transferred too soon to the alternative 

 plant host the individual often dies. It would no doubt be 

 easier to obtain results by preventing the normal migration. In 

 the case of the fir, where migration is sometimes deferred natu- 

 rally, at least for a percentage of the individuals, the cycle goes 

 back to its starting point. What nature has done in this instance 

 we should be able to imitate in other species by inclosing in 

 gauze bags on their plants the individuals about to migrate. 



It is important to note that the different parthenogenetic indi- 

 viduals of the same species that occur on the two plants may 

 differ enormously in structure in some cases, as shown especially 

 in the case on Hormaphis hamamelidis. The change must 

 be effected not by the new plant but by the original one; for 

 the parthenogenetic young may be fully developed before the 

 migration takes place, but whether their subsequent changes on 

 the new plant, that are sometimes very great, are brought about 

 by the new plant remains to be shown experimentally. 



LITERATURE, CHAPTER XX 



Balbiani, M. Le Phylloxera du Chene et le Phylloxera de la Vigne, etudes 



d'entomologie agricole. 1884. Mem. a I'Acad. des Sci. XXVIII. 



1884. 

 Blochmann, F. Ueber die Geschlechtsgeneration von Chermes abietes L. 



Biol. Centralbl. VII. 1887. 

 Ueber die regelmassigen Wanderungen der Blattlaiisenspeziell ueber den 



Generationscyklus von Chermes abietes L. Biol. Centralbl. IX. 



1889. 

 Bonnet, Ch. Traite d'Insectologie ou observations sur quelques espece 



de Vers d'eau douce et sur les Pucerons. Paris. 1745. 

 Burnett, W. I. Researches on the Development of viviparous Aphides. 



Proc. Am. Assn. Adv. Sci. 7th Meeting. 1853. 

 Castle, W. E. The Heredity of Sex. Bull, of the Mus. of Compar. Zool. 



at Harvard Col. XL. 1903. 

 Cholodovsky, N. Noch einiges zur Biologic der Gattung Chermes L. 



Zool. Anz. XII. 1889. 

 Weiteres zur Kenntniss der Chermes-Artem. Zool. Anz. XII. 1889. 

 Neue Mittheilungen zur Lebensgeschichte der Gattung Chermes. 



Ibid. XII. , 1889. 

 CoRNU, M. M. Etudes sur le Phylloxera Vastatrix. Mem. a I'Acad. de. 



Sci. Instit. de France, XXVI. 



