VIII APHIDAE 583 



or when the supply of food is less in quantity, or after a period 

 of deliberate abstention from food, sexual individuals are pro- 

 duced and fertilised eggs are laid which hatch in the spring, and 

 the phenomena are repeated. In other cases these phenomena 

 are added to or rendered more complicated by the intercalated 

 parthenogenetic generations exhibiting well-marked metamorphosis, 

 of kinds such as occur in apterous or in winged Insects ; while 

 again the habits of successive generations may differ greatly, the 

 individuals of some generations dwelling in galls, while those of 

 other o-enerations live underground on roots. 



Parthenogenesis. ■ — Eeturning to the various kinds of 

 peculiarities we have enumerated on the preceding page, we may 

 remark that the phenomena of parthenogenesis have been 

 thoroughly established as occurring in Aphidae since Bonnet dis- 

 covered the fact 150 years ago; and though they have not been 

 investigated in much detail it is known that the parthenogenesis 

 is usually accompanied by the production of young all of the 

 female sex. In other cases males are parthenogenetically produced ; 

 but whether these males come from a female that produces only 

 that sex is not yet, so far as the writer knows, established. A 

 note by Lichtenstein ^ suggests that usually only one sex is pro- 

 duced by a parthenogenetic female, but that both sexes are some- 

 times so produced. There is not at present any species of Aphid 

 known to be perpetuated by an uninterrupted series of partheno- 

 genetic generations. It was formerly supposed that there are no 

 males at all in Cliermes, but, as we shall subsequently show, this 

 was erroneous. It has, however, been observed that a series of 

 such generations may Ije continued without interruption for a 

 period of four years, and we have no reason to suppose that even 

 this could not be much exceeded under favourable conditions. 

 The parthenogenetic young may be produced either viviparously 

 or oviparously, according to species. 



Oviparous and viviparous reproduction. — The distinction 

 between these two processes has been extensively discussed, some 

 naturalists maintaining that they are thoroughly distinct ah 

 initio. This view, however, cannot be sustained. The best 



the course of events seems to be that in summer there e.xist only wingless and 

 winged parthenogenetic females, and that the sexually perfect forms appear for the 

 first time in autumn. 



^ MM. Schweiz. ent. Ges. iv. 1876, p. 529. 



