74 



plant until autumn. These are the progeny of the icinged migrant (see 

 612), and are the sole forms which injure the hop. 



613. HOP PLANT-LOUSE. 



Phorodon humuli. 



Return Migrant. 



This model represents the last generation produced on the hop, the 

 winged migrant form which in September returns again to the plum 

 and gives birth to three or more young, which are the true sexual 

 females, the first perfect sexual females produced in the cycle up to 

 this point. 



613a. HOP PLANT-LOUSE. 



Phorodon humuli. 

 Pupa of Return Migrant. 



This model represents the pupal stage of the return migrant. The 

 striking features are the wing-pads, which with another molt become 

 the ample flight organs seen in the model of the adult. 



614. HOP PLANT-LOUSE. 



Phorodon humuli. 



True sexual female. 



This model represents the generation born of the return migrant, 

 which never acquire wings and never leave the plum tree. Maturing 

 in a few days, according to the temperature, they are fertilized by the 

 true winged males which have been subsequently developed on the 

 hops, and have come from the hop fields to the plum. Shortly after 

 fertilization the winter eggs, with which the cycle started, are deposited. 



614a. HOP PLANT-LOUSE. 



Phorodon humuli. 



Young sexual female. 



This model represents the newly hatched stage of the only perfectly 

 developed sexual female produced in the life cycle of this insect. Eor 

 the adult of this stage see Exhibit No. 614. 



615. HOP PLANT-LOUSE. 



Phorodon humuli. 

 Winged male. 



This model represents the first and only male generation produced 

 in the life cycle of the Hop Plant-louse. This is developed in the 

 autumn and flies from the hop to the plum and fertilizes the true sex- 

 ual females. 



