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From the above account it will be seen that while the first three 
generations have an average life of nineteen days, the fourth to the 
twelfth followed each other at an average interval of precisely eleven 
days. Many of our observations show that a much earlier start and a 
more rapid growth are common, and that a greater number of genera- 
tions may consequently occur. ‘Thus, although in the above account the 
stem mothers hatched April 24, we have collected young of this genera- 
tion as early as April 10; and our breeding records show that adults 
of this first generation may mature and young migrants may be born by 
April 28, a ‘date more than a fortnight earlier than those used above. 
Adult (winged) migrants have been seen by us May 11—again more 
than a fortmght earlier than the corresponding date just mentioned. 
Further, we have had young of the third generation May 15, if we may 
trust an unverified report of Mr. F. W. Mally, who assisted me on this 
work in 1890. On the other hand, as I have counted generations from 
first-born to first-born, the hfe of a generation is here made too short 
for the average. 
No special attempt was made to determine the number of indi- 
viduals a single female may produce or the relative productiveness of the 
various successive generations. The fact is however worthy of record 
ran a single stem mother placed on a corn root in a breeding cage May 
4, brought forth her first young May 6 and her twelfth and last May 
15. At this time the first-born was a pupa, acquiring wings on the 19th. 
The stem mother lived until the 22d, and was then placed in alcohol. 
Another female of a midsummer brood brought forth fifteen young. 
According to the results of experiments conducted for the purpose | 
of determining the number of moults of the corn root aphis, and the 
intervals between successive moults, we find that this species moults 
four times, at average intervals of three or four days. Our most sue- 
cessful observations upon this and several other nice points of individual 
life history were made on isolated specimens, each placed upon the root 
of a potted plant which was then passed through a small glass tube and 
covered with earth except where the tube enclosed it. To prevent the 
escape of the plant louse the ends of the tube were lightly plugged with 
eotton-wool. 
Migration to Uninfested Fields —The last autumnal brood of the 
corn root aphis lives, so far as known, only upon roots of corn and purs- 
* As an effective illustration of the pressure which this plant louse species 
brings to bear upon the natural limitations on its increase, it may be worth while to 
say that assumin® the correctness of the figures here given as to the normal rate of 
multiplication and the number of generations produced in a year, and further sup- 
posing that all the plant lice descending from a single female hatched from the egg 
in spring were to live and reproduce throughout the year, we should have coming 
from the egg the following spring nine and a half trillion young. As each plant 
louse measures about 1.4 mm. in length and .93 mm. in width, an easy caleulation 
shows that these conceivably possible’ descendants of a single female would, if 
closely placed end to end, form a procession seven million eight hundred and fifty 
thousand miles in length; or they would make a belt or strip ten feet wide and two 
hundred and thirty miles long. Since the natural checks upon multiplication keep 
this insect species within limits of relative insignificance, the above statement would 
have no practical value if these checks were uniform in action. It is because they 
are highly variable that the aphis species, capable of reproduction at a rate so enor- 
mous, able consequently to take rapid and great advantage of any circumstance 
even slightly favorable, are a constant and often hideous menace to agriculture. 
autumnal generation appeared, belonging probably to the twelfth genera- a 
tion from the egg. ; 
