History and Habits of Pityogenes. 15 
more and more waste matter is pushed out through it, it 
becomes larger and more ragged. 
After feeding for a variable length of time, depending 
doubtless lar eely upon the weather, the young adults choose 
some especially warm day for their emergence. They do 
not appear to emerge in swarms as has been recorded for 
other scolytids by Hopkins (1899) and others, but seem to 
leave the trees singly or possibly a few at a time. The 
habit possessed by some bark beetles of “ swarming” during 
their migration from the old to the new host trees seems to 
be correlated with the possession of one or both of two other 
habits. In Dendroctonus valens Lec. which shows this 
tendency to a marked extent, the brood lives together in a 
common burrow during their larval and young adult, exist= 
ence and such a habit could develop much more readily than 
in a species like P. hopkinsi where the larvae each have a 
separate burrow and the feeding burrows of the young adults 
coalesce only occasionally and by accident. In cases where 
beetles attack living healthy trees, the habit of swarming 
is associated with the fact that a concerted attack upon the 
trees by a considerable number of insects is necessary in 
order to overcome the resistance of the tree and make it a 
suitable place for the development of the brood. 
There seems to be good evidence for the belief that the 
two sexes do not leave the tree together but that the males 
leave the old burrow several days earlier than the females. 
Evidence that this is true is furnished not only by laboratory 
observations but also by observations made in the field. 
After infested limbs brought in from freezing weather con- 
ditions had remained in the laboratory for a few days, 
males were seen to be emerging in considerable numbers and 
to be attacking the fresh pine limbs, while all but a few 
females still remained in the old brood burrows. In the 
field, careful observations were made during the early spring 
at intervals of a few days to determine the exact date at 
which the adults emerge and begin to enter the new host. 
These observations all show that in the field just as in the 
laboratory, the males emerge several days earlier and the 
