154 MISSOURI BOTANICAL GARDEN. 
There seems to be no doubt therefore that the band of the 
bag worm was responsible for the swellings, serving as a kind 
of ligature which prevented the passage of elaborated ma- 
terials from the outer part of the branch towards the 
trunk, and acting just as a wire would when wound 
around a growing twig. In the course of time the part of 
the branch above the band increased in size until such time 
as the passage of the elaborated material was prevented 
entirely, resulting in the final death of the branch. 
After attention had been called to the occurrence of the 
bags on arbor vitae and the girdling effect which they ex- 
erted, asearch of the various trees in the Garden and in 
the country around St. Louis showed that this action of 
the bag worm on arbor vitae was an extremely common 
one; in fact it was almost impossible to find a tree of ar- 
bor vitae which did not show one or more of the swellings. 
LIFE HISTORY OF THE BAG WORM. 
In order to understand the manner in which the insect 
kills the branch it will be necessary to refer briefly to its 
life history. The common bag worm ( Thyridopteryx 
ephemeraeformis Haworth*) hatches from eggs which have 
remained on the trees in the bags over winter. In the 
vicinity of St. Louis the larvae appear on shade trees in 
the early spring just as the buds are opening. The larvae 
feed on the young leaves, and very shortly after their ap- 
pearance begin to spin small bags which they carry about 
with them. As the insect grows older practically nothing 
of the larva is visible except the anterior portion of its 
body, the rest of its body being encased in the bag. 
Towards the latter part of the summer, from the beginning 
of August on, the larvae attach the bag to some twig near 
the outer part of the tree; they do this by spinning a band 
* For a good description of this insect and its habits, see Felt, E. P. 
Insects injurious to elm trees. (5th Annual Report, Fisheries, Game & 
Forest Commission of New York, p. 359, 1903). 
