fod 
Insects Bred from American Larch 27 
the fingers. No other insects were taken from this tree in 
the field and none were bred from it. 
Insect AssocraTions IN Larcn Woop Aanp Bark 
It is a well-recognized fact that in many cases certain 
species of insects not only live exclusively upon certain 
species of trees, but also that in many cases it is just as true 
that a certain insect is to be found only in a definite region 
of a tree. This, however, by no means holds for all species 
of bark or wood inhabiting insects, for many seem to attack 
indiscriminately any part of the tree from the trunk to 
branches an inch or even less in diameter, just as many insect 
forms attack a large number of tree species with no apparent 
preference. 
There are doubtless several factors which influence the 
choice by the insects of certain regions for breeding purposes. 
Perhaps the most important of these is the character of the 
bark, but actual height from the ground is an important 
factor in the case of some insects, especially such as are 
clumsy fliers. 
The character of the bark may apparently influence ovipo- 
sition in several ways. The actual thickness of the bark on 
the lower trunk of large trees undoubtedly deters many borers 
from ovipositing on account of the mechanical difficulty or 
even impossibility some find in piercing the thick outer 
layers and placing their eggs where the young on hatching 
will find the proper nourishment. Entirely aside from this 
factor of the thickness of the bark offering mechanical resist- 
ance to the oviposition of certain forms, the bark on the 
trunk has thicker layers of the edible and more or less fibrous 
and spongy inner bark, and this absorbs a greater amount of 
moisture and retains it longer. This maximum of moisture, 
while it offers conditions which are favorable or even neces- 
sary for the proper development of some species of borers, 
is just as truly unfavorable for other species. We shall 
presently see that certain borers are characteristically found 
in the thin-barked tops and limbs which in the next summer 
