126 



Winter Eggs. — Enlaro-cd model of the winter eggs, which are 

 attached by the sexual female to the terminal twigs of the plum, in 

 crevices around the buds. 



Stem-Mother. — The stout female plant-louse shown in this enlarged 

 model hatches from the winter egg and is characterized by shorter legs 

 and hone3'-tubes. It gives ])irth without the intervention of the male 

 to living 3"oung, of which three g-enerations are produced on the plum 

 trees, the last being winged and migrating to the hop. 



Winged Migrant. — This enlarged model represents the first winged 

 generation, the third produced on the plum. It instinctively Hies to 

 the hop plant and is the progenitor of from five to twelve wingless 

 generations of virgin females, which infest the hop plants until 

 autumn, and are the sole forms Avhich depredate on the hop. In 

 autumn the last generation again produces winged females which fly 

 to the plum trees. 



Hop-Affecting Stage. — This enlarged model represents the struc- 

 ture and appearance of the five to twelve wingless generations of vir- 

 gin females which people the hop plant until autumn. These are the 

 progeny of the winged rii.lgrants and are the sole forms which injure 

 the hop. 



Return Migrant. — This enlarged model represents the last gen- 

 eration produced on the hop, the winged migrant form which in Sep- 

 tember returns again to the plum and gives birth to three or more 

 3^oung, which are the true sexual females, the first perfect sexual 

 females produced in the cycle up to this point. 



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. 



True Sexuau Female. — Tliis enlarged model represents the gen- 

 eration l)orn of the return migrant, which never acquire Avings 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, 

 like those with which the (\ycle started, are deposited. 



Young Sj:xual Female. — This enlarged model represents the newly 

 hatched stage of the only perfectly developed sexual female produced 

 in the life cycle of this insect. 



Winged Male. — This enlarged model represents the first and only 

 male generation produced in the life cycle of the hop-plant louse. 

 This is developed in the autunm, and flies from the hop to the plum 

 and fertilizes the true sexual females. 



