FLUCTUATIONS IN MONTHLY MEAN TEMPERATURE. 125 
posed in the egg and larval states to a given degree of heat — this can only 
be obtained accurately by the daily records, but should be indicated by 
a comparison of the monthly means. This point is not considered int 
the discussion of these tables, but will be alluded to further on. 
Two conditions are absolutely necessary to excessive development j, 
first, that the parents shall be in a healthy condition and properly de- 
posit the eggs ; second, that the eggs shall very generally hatch out. 
There are other conditions that retard or favor, but these are absolutely 
necessary. It follows, therefore, that there are critical periods in the 
life of the insect, and hence it may be necessary, in order to ascertain 
the meteorological conditions favoring the development of excessive 
numbers in one year, to examine the meteorological records of the pre- 
vious years. 
Another fact to be taken into consideration in the discussion is that,, 
although excessive numbers may hatch, they may be killed, greatly 
diminished, or rendered diseased by unusual moisture ; or their devel- 
opment may be retarded, and migrating to a great degree prevented, by 
unusually low temperature. 
Bearing these facts in mind and remembering also that 1874 and 1S7& 
were the years of the great locust invasions, let us see if any important 
fact bearing upon the subject can be drawn from the foregoing table of 
the monthly temperature of the Northwest in the locust area. 
If we examine the fluctuations in the same month for the different 
years and select the extreme variations — that is, the difference between, 
the highest and lowest means — we shall find them to be as shown in 
Table VIII. 
Table VIII.- Extreme fluctuations in monthly mean temperature. 
Months. 
9 
U 
■3 
'S 
a 
© 
w 
V 
u 
M 
| Fort Sully. 
Yankton. 
Omaha. 
■M 
O 
V 
M 
Saiut Paul. 
| Leavenworth. 
*c 
ft 
C3 
"5 
03 
co 
Cheyenne. 
Denver. 
d 
o 
"3 
<x> 
ffl 
u 
o 
s 
a 
3 
a 
P4 
Virginia City. 
Bismarck. 
Average. 
o 

o 
o 
o 
o 
o 
o 
o 


o 
o 
o 
o 
January •. 
24.2 
18.6 
21.212.8 
17.4 
24. 9 18. 7 
10. 3 17. 9 14. 7 
25.1 
16.4 
21.7 
26.4 
19.3 
February 
35.3 
19. 2 33. 9|16.3 
i20.3 
33. 8 
17.9 
6.2 
8. 13. 
8.5 
30.0 
12. 5,33. 7 
20. G 
March 
38.8 
16.513.0,18.8 
17.7 
22.4 
15. 2 
10.0 
15. 4 10. 9 
23. 1 
29.7 
15. 0,27.3 
19. 6 
April 
13.6 
5. 5 10. 4 
10.8 
12.9 
13.610.2 
8.1 
9.1 
10. G 
6.7 
16.1 
3.3 
13.7 
10.3 
May 
9.0 
8.7 
7.9 
8.1 
6.9 
7.1 
5.0 
5.4 
8.7 
8.6 
6.8 
8.9 
7. 1 
6.1 
7.5 
June 
10.6 
10.6 
7.6 
5.8 
7.8 
9.4 
6.9 
6.4 
18.0 
6.1 
2.1 
10.4 
5.8 
2.4 
7.6 
July - 
10.4 
5.7 
6.4 
5.6 
4.2 
3.7 
6.5 
5.9 
8.3 
8.3 
5.3 
5.0 
3.9 
1.6 
5.8 
August 
4.0 
4.7 
8.8 
7.1 
7.1 
5.6 
8.2 
4.7 
6.0 
4.0 
0.4. 
3.3 
6.7 
3.5 
5.3 
September 
9.4 
7.9 
6.6 
6.6 
6.2 
12.5 
2.5 
2.7 
3.1 
2.9 
7.4 
9.4 
5.7 
6.8 
6 4- 
October 
6.9 
10.2 
4. 
5.5 
5.9 
8.1 
3.3 
6.2 
7.9 
9.4 
10.8 
6.8 
8.6 
2.0 
6.8 
November 
6.7 
10.7 
10.0 
7.9 
5.6 
8.6 
6.8 
8.1 
11.9 
9.7 
18.0 
14.6 
12.0 
11.4 
10.1 
December 
28.2 
15.5 
22.0 
20.2 
25.8 
25. 5 23. 2 
1 
6.8 
10.0 
15. 4 23. 1 
33.7 
13.8 
22.3 
20.4- 
Average variation of winter ami spring taken together, 16.8 (that is, from November to April). 
Average variation of summer and autumn taken together, 6.6 (that is, from May to October). 
In this table each number in a column shows the extreme variation, 
during the series of years included in Table VI, of the mean for the 
month opposite, and at the station mentioned at the head of the column. 
For example, in the Breckenridge column 24°.2 is the number given j. 
