12 BULLETIN 100, U. S. DEPARTMENT OF AGRICULTURE. 
An inspection of Table IV shows that the length of the life cycle 
of Generations IV to VII was almost the same. This is to be ex- 
pected, since in 1911 the months of June, July, and August had 
almost identical temperatures both day and night. It will also be 
observed that there was a very noticeable difference between the life- 
cycle periods of Generations VIII and IX, 16 individuals of the eighth 
generation averaging 15.4 days and 14 individuals of the ninth gen- 
eration averaging 33.4 days. The ninth generation thus required 
for development a period over twice as long as that required by the 
preceding generation, developing almost as slowly as the stem 
mother generation (see Table I). Yet the temperature during the 
daytime influencing the ninth generation differed but little from 
those which obtained during the development of the eighth. The 
probable causes of the slow development of the ninth generation lice 
is to be found in the colder night temperatures to which they were 
subjected and in the fact that the leaves at this time are becoming 
less vigorous and consequently afford poorer nourishment for the 
aphides than earlier in the season. There is a tenth generation, and 
in warm early seasons probably an eleventh, but in these generations 
the brood is small and the "lice" grow slowly. Plant lice may be 
found in early December giving birth to young, which are destined 
to perish either when the leaves drop or through exposure to hard 
frost. The author has observed dead aphides of all sizes on the 
brown frosted leaves during early winter. 
All the plant lice used for the life-history experiments were reared 
out of doors on young seedling walnut trees planted in pots and in- 
closed with glass cylinders. In 1911 the stock was procured from 
stem mothers collected on the earliest varieties of walnuts. When 
the work was started in 1911 it was too late to procure eggs, and so 
the data on the stem-mother cycle was acquired in 1912. 
After the ninth generation no more life-history experiments were 
carried on in the rearing cages, but a weekly examination was made 
of infested leaves in the field to determine the proportions of the dif- 
ferent forms, sexual and asexual, during the fall months. 
THE OVIPAROUS OR SEXUAL FORMS. 
The oviparous forms are the true sexes, comprising the winged 
male and the wingless oviparous, or egg-producing, female. The 
female aphis, after fertilization by the male, deposits true eggs, in 
which form alone the insect can tide over the winter months when 
no food supply is procurable. 
As is shown in Table IV, there is no real oviparous generation, for 
in all the later or fall generations a certain percentage of the young 
will develop either into the sexed males or the sexed females. On 
heavily infested trees oviparous aphides appear as early in the season 
