Sex Determination in Phylloxerans and Aphids 293 
internal factor, that preponderates especially in those cases where 
a large number of eggs of one kind are produced. ‘These numbers 
are so large that the results cannot be dueto chance. ‘The nature 
of the factor involved will be discussed later. 
THE SEXUAL FORMS OF OTHER SPECIES 
The sexual forms of only a few of our Phylloxerans have ever 
been seen. Pergande describes 4 out of a total of 33 species. 
The minute size of the sexual individuals, as well as the fact that 
the eggs from which they come are often deposited on the limbs or 
trunk of the tree where they are difficult to detect accounts in part 
for this lack in our knowledge. By the following simple method, 
however, it is very easy to obtain the third generation. ‘The galls 
should be collected when the winged migrants are ready to emerge 
and placed in a glass dish covered with fine cloth. The water 
from the galls may collect on the glass and drown the migrants 
unless special precautions aretaken. [use large flat dishes and turn 
them upside down so that the galls lie in the middle of the cloth. 
At other times I have put the winged individuals into paste-board 
boxes, covered with cloth. ‘The eggs are deposited on the glass, 
cloth, galls or paste-board, as the case may be. In the course of 
about a week they hatch; the small, numerous males will be found 
running rapidly about, while the females move more slowly. 
Having found that the sexual forms in P. fallax and P. carye- 
caulis are mature on hatching, I was led to hope that mature sexual 
forms of all the other species might also be obtained, by the method 
given above. To my surprise it was found that in three other 
species studied all the eggs laid by the migrants were the same size— 
not of two sizes as in the other two species. ‘Thinking that I might 
have obtained eggs of only one sex, [opened large numbers of individ- 
uals and still found eggs of only one size. [ then made sections 
of the individuals hatched in my dishes and found that the sexual 
organs were entirely immature. Furthermore, carefulstudy of the 
individuals showed that they are all alike, or so similar that I 
detected no differences by means of which they could be separated 
into males and females. 
