LIFE HISTORY. 
13 
reproduction of the winged migrants of both the spring and the fall 
generations. The stem-mothers are very prolific, as shown in Table 
III, one aphis being capable of populating several leaves in a very 
short time. 
NUMBER OF GENERATIONS ON ALTERNATE HOST. 
Mr. Franz Kemisch, of Saaz, Bohemia, who observed the emergence 
from the winter eggs, obtained two generations on plum. The writer 
did not observe the emergence of the first generation, but during the 
spring only two generations were found on prune, the second one 
being winged. In Bulletin 160 of the California experiment station 
Prof. W. T. Clarke reports the 
appearance of winged aphides 
14 days after the first wingless 
insects were observed. This 
would be sufficient time for 
but one generation to mature, 
and it is very probable that 
there are only two generations 
upon the prune in the Pacific 
region. 
MIGRATING FORMS. 
The winged or migrating 
aphides, except for the pres- 
ence of two pairs of relative^ 
large, delicate wings, some dark 
spots on the thorax, and a 
slightly more slender body 
(fig. 3^ and PI. I, fig. 1) differ 
little from the wingless form. 
They appeared at Perkins, 
CaL, May 15 and were present 
there and at Santa Kosa until June 15. Five belated individuals 
were observed at Perkins the latter part of July, but the migrations 
had taken place by the 20th of June. 
Fig. 2. — The hop aphis (Phorodon Jiumuli): Wingless 
viviparous female. Greatly enlarged. (Original.) 
MIGRATORY ACTIVITIES. 
These winged aphides, which mature on the plum, are the first 
migrants. They are weak fliers, but when aided by a light wind 
may travel some distance. Their progeny, the wingless viviparous 
females, which are the most common forms found upon the hopvines 
throughout the season, are incapable of migration from one vine to 
another except where two vines come into contact so that the aphis 
