5 84 HEMIPTERA-HOMOPTERA CHAP. 



authorities are agreed that in the earliest processes of individual- 

 isation the ovum, and the pseudovum l giving rise to a viviparous 

 individual, are indistinguishable. Leyclig, Huxley, Balbiani, and 

 Lemoine are agreed as to this. Nevertheless, differences in the 

 development occur extremely early. The nature of these differ- 

 ences may be briefly described by saying that in the viviparous 

 forms the embryonic development sets in before the formation of 

 the egg is properly completed. Balbiani says, " In fact at this 

 moment [when the viviparous development is commencing] the 

 germ [pseudovum] is far from having obtained the development 

 it is capable of, and from having accumulated all the matter 

 necessary for the increase of the embryo, so that the evolution 

 of the former coincides, so to speak, with that of the latter. On 

 the other hand, in the true ovum the two processes are chrono- 

 logically separate, for the rudiment of the new individual never 

 appears before the egg has completed the growth of its constituent 

 parts." As regards the difference in structure of the organs of 

 viviparously an,d oviparously producing individuals, it is sufficient 

 to remark that they are not of great importance, being apparently 

 confined to certain parts remaining rudimentary in the former. 

 Leydig, indeed, found an Aphis in which certain of the egg-tubes 

 contained eggs in various stages of development, and others 

 embryos in all stages. 3 



As regards the physiology of production of winged and wing- 

 less individuals there has been but little exact inquiry. Vast 

 numbers of individuals may be produced without any winged 

 forms occurring, while on the other hand these latter are occa- 

 sionally so abundant as to float about in swarms that darken 

 the air ; the two forms are probably, however, determined by the 

 supply of food. The winged forms are less prolific than the 

 apterous forms ; and Forbes has noticed in Aphis maidi-radicis, 

 where the generations consist partly of apterous and partly of 

 winged individuals, that when the corn begins to flag in conse- 

 quence of the attacks of the Aphis, then the proportion of 



1 The term pseudovum is applied, as a matter of convenience, to the earlier condi- 

 tion of the viviparously- produced form, and the term pseudovarium to the ovary 

 producing it. 



2 Balbiani, Ann. Sci. Xat. Zool. (5) xi. 1869, p. 29. For concise recent re- 

 marks on the early embryonic states, see Lemoinr, Bull. Soc. cut. France, 1893, 

 p. Ixxxix. 



3 Ada Ac. German, xxxiii. 1869, Xo. 2, p. 81. 



