966 
Journal of Agricultural Research 
Vol. V, No. 21 
it is possible that continued cold weather, or a succession of freezings, is 
essential. In either case it seems probable that the embryo must have 
experienced a sufficient amount of low temperature long before spring 
and that it must thereafter continue to remain dormant till the proper 
average temperatures exist for its renewed activities. If, however, the 
embryos be subjected to temperatures well above the critical at any 
period before they have revolved, this change is fatal to them. 
What this critical temperature is, can not be determined with any 
exactness from the data at hand. In 1915, from March 8 to 16, the period 
during which growth was resumed, the average temperature dropped to 
34 0 F. only once, and it was below 36° only twice in the week. In 1914, 
however, the averages varied between 18 0 and 6o° during what appears 
to have been the critical period, although from March 14 to 18, inclusive, 
it was above 34 0 . It seems probable that the critical temperature is 
close to 36°. Apparently, also, this critical temperature, or average 
temperatures a little above it, must continue for a period of some days, 
since frequently average temperatures higher than the critical occur for 
one or sometimes more days in January and February without affecting 
the insects. 
It is interesting to note that eggs of A . avenae brought into the green¬ 
house during the winter hatched normally. Eggs of this species fre¬ 
quently hatch on the trees after warm spells of two or three days’ duration 
in January and February; and while the writers have not as yet made a 
thorough study of the embryology of this species, yet during the winter 
they have taken several eggs in which the embryo had revolved. 
These observations are of particular interest, since they undoubtedly 
explain the fact stated by several writers that a very low percentage— 
about 2 per cent, according to Gillette and Taylor (14, p. 24)—of the 
eggs of A . pomi hatch. 
Hatching. —The first eggs hatched in 1914 about April 8 and the 
last about April 25. At this time nearly all the buds showed some green 
and in many cases the tiny leaves were free from the bud scales. Since 
it is as immature stem mothers that this and corresponding species are 
usually treated with insecticides, it will be well to include here a com¬ 
parison of their dates of hatching. In the spring of 1914, at Vienna, 
A . avenae commenced hatching on March 28. A . malifoliae and A . pomi , 
however, did not hatch until about April 8. A few eggs of A . malifoliae 
hatched before that date, and this would seem to indicate that the rosy 
apple aphis is perhaps slightly earlier than the green aphis. For all 
practical purposes, however, their hatching dates are the same, while 
that of A. avenae is very much earlier. 
The young stem mother emerges from the egg head foremost, and the 
latter is always split evenly over the vertex of the insect. This is accom- 
