THE GENUS DENDROCTONUS. 19 
In species 3 there is but one generation annually in the more 
advanced broods, while the more retarded ones may pass through 
two winters before they complete their development and emerge. 
The adults begin to emerge, attack other trees, and deposit eggs 
toward the last of June, but the broods do not develop before hiber- 
nation begins in the fall. 
Species 4 has two or three generations annually in its northern 
range. In the intermediate range, represented by Try on, N. C, there 
are three or four generations, while in the more southern range there 
may possibly be five generations, with activity continuing during the 
warmer days of winter. Under average conditions the first attack 
is made about the middle of May, and under favorable conditions the 
resulting brood develops to adults and emerges in about sixty clays. 
Species 5 evidently has a seasonal history similar to species 2 
In species 8 there is but one generation annually. The attack 
begins in June, and the broods do not emerge until the following June 
to August, or later. 
In species 9, 10, and 11 there is but one generation annually, and the 
seasonal history of each is quite similar to that of the others. The 
first attack is made during the last week in July or first week in 
August, and the broods do not develop and emerge until the following 
July and August. 
In species 12 and 13 there is a single generation annually. The 
first attack is made in April to May, and the broods emerge the fol- 
lowing April to July. 
In species 14 and 15 there is but one generation annually. The 
first attack is made in June, and the broods do not emerge until the 
following June to August. In species 17 the attack begins a month 
earlier. In the European species (No. 21) the first attack is made 
in May and June, and the broods emerge the following May to August. 
In species 22 and 23 there is but one generation annually. The 
attack is made during the first warm days in March to April, and the 
broods emerge the following March to September, or later. 
INFLUENCES OF LATITUDE AND ALTITUDE ON SEASONAL HISTORY. 
The beginning and ending of the hibernating period vary somewhat 
among the different species, and in each species there is considerable 
difference at different latitudes and altitudes within its range. 
Within the area of a given State or section of the country this differ- 
ence in the beginning or ending of a given period in the seasonal his- 
tory of a species can be estimated after the date of beginning is deter- 
mined for a given season in a given locality. In the spring of a given 
year the average difference in the time of beginning activity, emer- 
gence, flight, attack, etc., at the same altitude, will not vary much 
from four days later for each degree north, or four days earlier for 
