AUTUMN NUMBER D7 
A little later the experiment was repeated. Colony F was 
placed on another uninjured tree and colony G on a small tree 
which was girdled in a manner similar to the tree in the previous 
experiment. Colonies H and I were checks on uninjured branches 
of another tree. These were carried on as the previous experi- 
ments for several weeks with no decided results on wing produc- 
tion by injuring the host plant. The checks produced winged aph- 
ids about as consistently as did the injured limbs. Check colony E 
appeared for a time to be an exception to this rule, and it was 
thought that the tree on which they were living might be the 
cause of this. Consequently some of the apterous aphids from 
colony EF were taken out and three new colonies started from 
them. Colony J was placed on a branch near A and B on tree 
No. 1, which had produced winged forms; colony K was placed 
on a limb of the girdled tree No. 4 by the side of colony D which 
had also produced winged forms; and colony L was moved to a 
new limb of tree No. 5, near colony E, for a check and to counter- 
act any effect of moving the aphids to other trees. As the results 
show, winged aphids were produced not only in each of these 
three colonies, but also a little later in the original colony E. 
About two months after being girdled one of the trees was 
turning very yellow from the effects. Therefore, two colonies of 
aphids were placed in cages on limbs of this tree, and two other 
colonies were placed on limbs of a healthy tree, as a check. All 
four colonies were taken from the same colony containing both 
winged and apterous forms. Here, also, during two months’ 
time, winged forms were found in the checks in about the same 
ratio as in the colonies on the girdled tree. Therefore, the un- 
healthy condition of a pine tree due to girdling or any change 
which it may cause in the sap of the tree will not cause the aphids 
on it to produce wings in increased numbers over those on 
healthy trees. 
Effect of Crowding on Wing Production. In many colonies 
it was observed that no winged forms appeared until several 
days after the colony was started, or until it had increased in 
size and often covered the limb thickly. Consequently the ques- 
tion arose as to whether or not the large number of aphids there 
caused the production of wings, either directly, or indirectly, by 
lessening the food supply. To investigate this point two colonies 
were started on a girdled tree. The first one, colony M, was 
placed on a limb of this tree on December 11 and allowed to 
