INFLUENCE OF TEMPERATURE ON DIFFUSION. 93 
stances at a slightly lower temperature, but these instances are 
probably too infrequent to become of economic importance. 
With the eggs in the North the case may be more important, because 
these, deposited in dead leaves of bluegrass, and sometimes probably 
buried under several inches of this matted grass, with the living leaves 
covering this over, the temperature and moisture would both be 
greater than at several feet above ground without such protection. 
Mr. Philip Luginbill of this bureau in April, 1911, proved this to be 
true. He placed a thermometer in just such a position as men- 
tioned above, in a protected nook where the sun could shine directly 
on it in the grass and no wind could reach it and found that the 
temperature was 10° to 12° F. higher than when the thermometer 
was several feet above the ground and in the shade. The junior 
author has found that eggs are deposited in just such places, and that 
hatching takes place in spring at a temperature ranging, as recorded 
by the thermograph, from 32° to 62° F. It would appear that 
eggs deposited in a position as mentioned above would hatch sooner 
than those deposited in places where the temperature would not be 
so high and the stem mothers from the former would reproduce, 
the pest becoming more abundant in the spring and making its way 
from grass to grain earlier and in greater numbers than they would 
from the cooler locations. 
This leads us to a very interesting and important point in tem- 
perature effects on the species. In the South, seemingly south of about 
latitude 35° to 36° north, it has been impossible to find eggs of this 
and other species of aphidids in the fields. There is in the perpetua- 
tion of the species no apparent need of this stage, however, as it is 
able to continue throughout the entire year reproducing viviparously. 
In the North this is probably not possible except during very mild 
winters. The situation is therefore about like this: Gradually as 
we proceed southward from about latitude 38° the sexual forms and 
eggs disappear, while to the north of about latitude 36° hibernation 
is confined more and more to the egg stage, until this becomes ex- 
clusively the state in which the winter is passed. 
The practical, economic importance of this is that there is con- 
siderable doubt relative to the amount of injury the pest would 
cause north of this belt of country if there were no Toxoptera drifting 
in from the south. In other words, but for the countless myriads 
developing south of this belt and sweeping over and beyond it, there 
would be few if any destructive ravages. If this is the true state 
of affairs, the oats crop north of this belt is to a certain degree de- 
pendent upon the success or failure in controlling the pest in Texas, 
Oklahoma, New Mexico, and South Carolina. 
Summarizing, then, it would appear from the information we have 
been able to obtain, and which is given throughout this publication, 
