126 MISC. PUBLICATION 657, U. S. DEPT. OF AGRICULTURE 
FIGURE 23.—Galls caused by Pachypsylla venusta on hackberry. 
Famity APHIDAE 
In the aphid family only the sexually perfect females lay eggs, and 
the parthenogenetic females give birth to developed young, which may 
in same cases be enclosed in a pellicle. The wing venation differs 
from that in the Chermidae, the next family to be discussed, by having 
the radius of the forewings branched and the outer part of the stigma 
bounded by R,. 
One of the characteristics of the Aphidae is that of cyclic repro- 
duction, in which a series of parthenogenetic generations is alternated 
with a sexual generation, and there may be an alternation of winged 
and wingless forms within the series of parthenogenetic generations. 
Also, the life history may be further complicated by having part of it 
occur on one species of plant (the primary host), and the remainder 
on an entirely different species or genus (the secondary host). In 
such cases the life history may be somewhat as follows: From an over- 
wintering egg on the primary host, the stem mother hatches in the 
spring. This stem mother gives birth to living young, all females, 
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