CONSTRICTION OF TWIGS BY THE BAG WORM. 150 
around the branch in such manner that when the bag is 
finally fastened it is suspended from the branch, as shown 
in plate 21. The band varies in width and in thickness. 
It is closely adapted to any irregularity in the bark of the 
particular twig, and is so wound around the twig that it 
can be moved laterally only with difficulty. It is in no 
way pasted or glued to the bark, as can easily be shown 
by cutting the band at any point, which will cause the 
cocoon to drop. The insect will attach its bands, almost 
without fail, only on one-year-old twigs. Whether this is 
due to the fact that it is upon these twigs that it has been 
feeding, or whether it is due to the fact that these twigs 
are usually smallest, I have not been able to determine. 
That the size of the twig has something to do ‘vith the 
weaving of the band is shown by the following experiment : 
In order to test the strength of these bands, the writer de- 
sired to get bands of as great a length as possible. A 
number of the larvae which were found ready to attach 
themselves to twigs were taken and placed on twigs having 
a diameter of about three-eighths of an inch; in no case 
did these transplanted larvae weave a band around a twig of 
this size. When they found that they could not get away 
from the large twig they made a band which they glued to 
the lower side of the twig, thereby suspending the cocoon; 
when the branch was one-fourth inch thick or less they 
invariably made a band completely around the twig. The 
relation between the size of the twig and the number of 
bands is shown in detail in Table I. After the band has 
been woven, the cocoon hangs on the tree over winter and 
into the next spring, in fact until about the end of June of 
the following year. Some six weeks or more after the leaves 
have fully opened one can pass under trees which have been 
covered with the bag worms during the winter and find 
hundreds of them lying on the ground under the trees. 
The falling of the bags is brought about by the bursting of 
the bands by the twigs as they increase in diameter. That 
