Feb. i6,1924 Effect of Eight Exposure on Migration in Aphids 
515 
day for the northern part of Scotland reveals a drop of 4 hours from the 
long days of 18.2 hours in early July to 14.2 hours in the latter part of 
August. 
Several species of plant lice, such as the spring grain aphis, cabbage 
aphis, and others, are known to pass the winter in the egg stage in the 
more northern climates, whereas farther south reproduction takes place 
throughout the winter without the presence of sexes. Such being the 
dase, one is tempted to conclude that the northern temperatures stimulate 
the formation of the sexes; yet Baerg (7), working with Myzus brevi- 
pilosus, found that no sexes were produced in Arkansas, but when taken 
to New York sexes were produced abundantly in a warm laboratory in the 
winter. In this connection it should be remarked that along with the 
higher temperatures of the more southern latitudes the variation in the 
length of day is not so striking, and evidently not sufficient to stimulate 
egg production. 
An extended study of the appearance of the sexual forms was made by 
Davidson (5), working with Aphis rumicis. He remarks that “on the 
approach of autumn, the influence of some important factors results in 
the development of sexual forms irrespective of the number of genera¬ 
tions. There would appear to be at work some inherent tendency asso¬ 
ciated with adaptation to seasonal conditions. As autumn advances, 
an increased number of sexual forms appear. * * He concludes that “ sexual 
forms develop under favorable food and temperature/* and that “ they 
were produced under the low temperatures of October in the glass-house, 
and later under the warm temperature in the greenhouse. Further, 
agamic and sexual forms appeared together under the same conditions 
of food and temperature. The autumn seasonal factor would appear 
to be important in accounting for the large number of sexual forms pro¬ 
duced at this period.** It is evident that Davidson recognized an au¬ 
tumnal factor, and had he used the factor of the length of day, his results 
would doubtless have been more in harmony with the facts. 
It is generally recognized that the same insect differs in habits in differ¬ 
ent sections, necessitating different methods of control, but in some cases 
no adequate reason has been given. Lathrop (13) discovered that the 
apple aphids in Oregon differed considerably in their habits from those in 
New York. Hatching of the eggs of Aphis sorbi begins in Oregon about 
March 20, and is over a month earlier than in New York.. The spring 
migrants leave for plantain in New York regularly in the third generation, 
and sometimes in the fourth, whereas, in Oregon the migration is drawn 
out and tends to be irregular. The fact observed by Baker and Turner 
(2), that migrants appear in increasing numbers up to the eighth genera¬ 
tion in Virginia, which is further south, and where the length of day is less 
variable, would indicate that length of day, in relation to the time of 
hatching, is the determining factor. At the time the eggs hatch in 
Oregon the days are still short and do not offer the stimulus for migra¬ 
tion. Furthermore, Lathrop’s ability to keep Aphis sorbi on apple 
until November, by shading with cheesecloth, and the writer*s experience 
in rearing 10 generations on apple by June, with the possibility of a still 
greater number, emphasize the importance of light in the migration of 
aphids. 
The importance of knowing the factors that influence the production 
of migratory forms is clearly evident with Aphis sorbi , for should the 
fourth generation be produced on apple before migrating, extreme 
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