Insects Bred from American. Larch 41 
with D. simplex near its base, but as the P. rufipennis became 
more numerous farther from the base the D. simplex became 
less abundant and above twenty feet none were to be found. 
In this tree no pitch tubes were present showing that the 
tree was either dead or in a very weakened condition at the 
time of the entrance of the beetles. 
Adults of D. simplex were taken from their burrows in 
the field on April 5 and April 22. They were taken from 
the breeding cages under natural conditions on May 25 and 
26. Swaine records (1910, p. 81) finding the egg-tunnels 
containing eggs in the outer ends and larvee of all sizes 
boring in the bark on July 10. On August 6 these tunnels 
were occupied by grown larve. Pup, recently transformed 
adults as well as emergence holes were present. Eggs of a 
second brood were found as late as August 26. This varia- 
tion in life history is no more than should be expected. No 
dogmatic statements concerning the seasonal history of an 
insect can safely be made, for “life cycles are more subject 
to variation than are structural details. It would seem to 
be axiomatic that a physiological process should be more 
readily altered by unusual environmental influences than 
would be anatomical structure, yet some of our entomologists 
speak of the seasonal histories of insects as if they were 
immutable. 
D. simplex is usually not preceded in larch by any other 
insect but in some cases may be. Trees injured by blaze 
sears or by abrasions often attract Serropalpus barbatus. 
Diseased or weakened trees also may be infested by Asemum 
moestum or Melanophila fulvogutta even before the entrance 
of the brood of D. simplex. As we have already seen P. 
rufipennis is a bark beetle often associated with D. simplex 
in the bark. 
The predator Phyllobaenus dislocatus is often found not 
only in the burrows but also is frequently seen actively 
scurrying over the bark of infested trees. Spathius tomer 
and an undetermined chalcid have been bred from material 
containing both D. simplex and P. rufipennis, The former . 
of these is certainly parasitic upon P. rufipennis and may 
well parasitize D. simplex also. 
