Journal of Agricultural Research voi. xxvn, no . 7 
5H 
tions are produced on the secondary host, and with the approach of 
autumn, a winged generation is again produced that flies back to the 
original, or primary, host, where the true sexes are produced. In place 
of the migatory form to a secondary host there is sometimes produced 
a summer resting stage, or dimorph, as in the boxelder aphid. 
DISCUSSION OF THE LITERATURE 
A glance at the literature indicates that many explanations have 
been offered for this autumnal appearance of the fall migrants and the 
sexes, such as a failing food supply, adverse conditions, temperature, 
and seasonal conditions. Just what the seasonal factor is, such as the 
tength of day, has not been discovered, and it is interesting to note 
that the literature itself records the appearance of the sexes earlier in 
the fall in the more northern latitudes, where the shortness of the day is 
more pronounced. 
Smith (16, p. 43-44) in working with the clover aphis (Anuraphis 
bakeri), noted in Idaho that the ‘'fall migrants were found oii fruit trees 
on August 21, and by September 2 they were numerous. Yet the mean 
temperature for August in this locality was 69° F., which was the same as 
the mean temperature for July and 4.5 0 F. above the mean temperature 
for June.” Smith further remarks that "temperature changes incident 
to the approach of winter may not be the chief factor serving to stimulate 
the development of fall migrants. As further evidence in this connec¬ 
tion, clover plants artificially infested in late summer and transferred to 
warn* rooms in the county courthouse at Twin Falls, developed males and 
winged females in great abundance during the following February and 
March.” 
Tabi,® I. —Mean temperature records and length of day for Knoxville , Tenn., and the 
northern part of Scotland 
Knoxville, Tenn., 
latitude 36°. 
Sutherland, Scotland, 
latitude 58° 18'. 
Tempera¬ 
ture. 
Tength of 
day. 
Tempera¬ 
ture. 
Tength of 
day. 
January. 
°F. 
38.8 
Hours. 
IO. 0 
°F. 
38 
Hours. 
7 • 1 
February. 
41.9 
IO. 9 
38 
9.4 
March... 
48. 7 
II. 9 
40 
II. 7 
April... 
58.0 
13 * I 
44 
14*4 
May. 
67. 2 
14 * I 
48 
16.7 
June.... 
74*0 
14. 6 
53 
18. a 
July . 
77. 1 
14.4 
56 
17.6 
August. 
74 * 7 
13*5 
56 
15-4 
September. 
70. 6 
12.4 
52 
12.9 
October.. 
59 - 9 
11. 3 
48 
io. 4 
November. . 
47*9 
10. 2 
42 
7.9 
December. 
40.3 
9 * 7 
39 
6.4 
Similarly, Jackson (12) records the presence of oviparous females and 
eggs of several species in Scotland during the first part of September. No 
doubt the sexes were already present in August if eggs were found early in 
September. A study of the temperature records for Scotland, as sum¬ 
marized in Table I, shows that July and August have about the same 
mean temperature of 56° F. On the other hand, a study of the length of 
