Plant-Lice 



fly to the hops, settle upon the leaves, and begin to give birth 



to the fourth generation which is composed like the second 



of wingless parthenogenetic females 



and the same process is repeated until 



possibly eleven or twelve generations 



in all have been produced. By the 



time the twelfth generation makes its 



appearance the month of September 



has come and hop-picking is well along. 



All of the lice at this time acquire wings. 



Some of them may be of the twelfth 



generation, others through the fact that 



their ancestors have been late born 



through a series of generations may be 



only of the fifth generation. The first 



to acquire wings in the autumn are 



always females and these fly back to the 



neighboring plum trees. Later individuals of this generation 



and frequently all of the individuals of an additional generation 



Fig. 164. Phorodon hu- 

 muli: egg-laying female. 

 (From Insect Life.) 



Fig. 165. Phorodon humuli: migrant. (From Insect Life.) 



on the hop are true males, the male thus making its appearance 

 for the first and only time in the life round of the species. By 



267 



