28 College of Forestry 
after the death of the tree appear to be absolutely dry, but 
which nevertheless apparently offer conditions which are 
ideal for certain two-year forms. Moisture conditions dur- 
ing the second summer in the trunk and in the thinner barked 
limbs is so extremely different that one would hardly expect 
to find any forms in common between them. As a matter 
of fact this expectation is nearly realized, for of the two- 
year forms, or of forms occurring under the bark during the 
second summer after the death of the tree, only two species. 
were bred both from the limbs and from the lower or middle 
trunk. 
The Lower Trunk in Dying or Recently Killed Larch Trees. 
The trunk region itself can be subdivided into two or more 
regions or habitats upon the basis of the insects found. 
therein. In the dying or recently killed trees Dendroctonus 
simplex is perhaps the most characteristic bark beetle inhab- 
itant of the lower trunk. It was not found in the bark at a 
greater distance than twenty feet from the ground and was 
most numerous in the lower ten feet. In felled trees, how- 
ever, D. simplex occurs throughout the trunk even around 
the bases of the branches. Apparently, then, the lmiting 
factor here is distance from the ground, and doubtless the 
clumsy build of the beetle and its rather poor powers of flight 
are responsible. 
Another bark beetle often found in the lower trunk is. 
Polygraphus rufipennis. It breeds in all regions of the trunk 
and even in the tops and larger hmbs. It is worthy of note 
that when it occurs in the same tree trunk as D. simplex it 
is much less numerous in the lower regions of the trunk 
where the latter species occurs, than it is in the middle and 
upper trunk. This is not true of trees not infested by D. 
simplex. The explanation of this seems apparent. The 
Dendroctonus enters the tree slightly earlier than Poly- 
graphus, which on finding the lower trunk already occupied 
by numerous broods of the other species, seeks other parts 
of the tree to construct its brood burrows. In trees infested. 
by both it is interesting to note that as we go farther and 
farther from the ground the burrows of D. simplex become 
