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 soxed females. On 

 heavily infested trees oviparous aphides appear as early in the season 



