480 THE INVERTEBRATA 



The following summary will assist in the understanding of the life 

 cycle of Aphis rumicis : 



Fertilized eggs laid in autumn 



I 

 Viviparous parthenogenetic females I Euonymus 



Winged migrant parthenogenetic females 



^ . . . 



Wingless parthenogenetic viviparous females 



I !- Vicia {aha 



Winged viviparous females (autumn) J 



I 

 Winged males x Wingless oviparous females] 



\ \ Euonymus 



Eggs laid in autumn J 



The cyclical reproductive phenomena in aphides as just described 

 raise important problems relating to the intrinsic differences between 

 sexual and parthenogenetic individuals, and to the environmental 

 conditions governing the occurrence of these phases in any life 

 cycle. 



Fertilized eggs produce only strictly parthenogenetic females. These 

 multiply by diploid parthenogenesis, i.e. the eggs retain the full 

 complement of chromosomes and are not capable of fertilization. 

 Eventually come individuals capable of bearing sexual forms, sexu- 

 parae. The sexual forms arising from these produce germ cells 

 undergoing normal reduction and which are therefore haploid. 

 It follows then that fertilisation will restore diploid partheno- 

 genesis. Sexual differences are indicated in the chromosomes; the 

 female of Aphis saliceti possessing six, of which two are sex chromo- 

 somes; the male only five, one only being a sex chromosome. Sexual 

 reproduction leads however only to the production of parthenogenetic 

 females and not to males and females in equal numbers, as might 

 be expected. This appears to be due to the fact that in the maturation 

 of sperms, those with only two chromosomes die. Fertilization there- 

 fore is always between sperms and ova each with three chromosomes, 

 of which in each case two are normal chromosomes {autosomes) and 

 one is a sex chromosome X. The capacity of females with six 

 chromosomes to produce male offspring with only five is due to the 

 fact that in the maturation of male-producing parthenogenetic eggs, 

 reduction in the number of chromosomes only affects the sex {X) 

 chromosomes, one remaining in the tgg^ the other going to the polar 

 body. In this way a parthenogenetic female with six chromosomes, 

 i.e. 4 + XJ\r, gives rise to males with only five, i.e. 4 + X. 



