Xos. 1)22-023] THE SEXES IN THE APHID 



515 



generation. It is impossible to state what the fifth gen- 

 eration would have included, since only one fifth of the 

 probable progeny were produced or survived. After the 

 fifth generation up to the complete disappearance of vivip- 

 arous forms, there was again a replacement— this time 

 rather sudden than gradual— of the wingless females by 

 winged ones. The same gradual disappearance of wing- 

 less viviparous in favor of winged females was observed 

 in several other experiments of shorter duration which 

 are not included in this paper, and has also been found 

 in Microsiphum destructor by Miss Gregory (1917). It 

 is therefore to be regarded as of general occurrence. 



2. There is observed in two of the tables (IV and VI) 

 a gradual increase in the tendency of wingless females to 

 produce sexual females instead of males, as they most 

 often do when the sexual phase begins. Thus in Table 

 IV, generation IV, 2.5 per cent, of the sexual forms pro- 

 duced by apterous parents were sexual females. In gen- 

 eration V, 12.9 per cent, of the sexual forms were females. 

 In the sixth generation, of the sexual offspring of apter- 

 ous parents, 29.7 per cent, were females. In the seventh 

 generation, which is the last from apterous parent-. 73.3 

 per cent, of the sexual offspring were females. Thus, 

 while the apterous parents produced mostly males during 

 the sexual phase, there is a gradually increasing tendency 

 to produce females. In Table VI is a brief indication of 

 this same phenomenon. Males alone (of the sexual indi- 

 viduals) appear in the seventh generation, but a small 

 number of females in the eighth generation. Unfortu- 

 nately no apterous parents were available for a further 

 generation. If it were possible to obtain wingless 

 females in Inter generations it would be interesting to 

 note whether they would not eventually produce only 

 females. 



Whether there is a similar progressive change in the 

 sexual offspring of winged females is not so clear, since 

 in none of the last three tables of this paper are there 

 any male offspring of alate parents. However, in the 



