70 
BULLETIN 903, U. S. DEPARTMENT OF AGRICULTURE. 
All the individuals utilized in this experiment were reared on 
severed pieces of roots in a petri dish under cellar conditions. In- 
dividuals 19 and 20 lived on a much poorer root than the others, and 
thus is explained the relatively slow growth (34 days) of the one 
and the early hibernation of the other. Individuals 4 and 14 moved 
away after their first and second molts, respectively. It will be 
observed from the summary that the average period of the first instar 
(13.3 days) is considerably longer than is that of the three succeed- 
ing instars combined (7.9 days). The comparative periods of the 
instars are about constant; that is, an individual with a short first 
instar will have short succeeding instars and one with a long first 
instar will have long succeeding instars. 
The records of Table XX were made in midsummer at a tempera- 
ture of 68° F. In the soil at such a time of the year the temperature 
is higher and the development of the phylloxera more rapid, while 
in spring and late fall the development is correspondingly slower. 
The developmental period of the hibernated larvse varies greatly, 
not so much from temperature as from other causes. There is an 
average period of two and one-half weeks from the commencement 
of feeding to the shedding of the first skin, and after that an average 
period of three weeks between the casting of the first skin and the 
shedding of the fourth, the second, third, and fourth instars occupy- 
ing an average space of a week each. As summer progresses the de- 
velopment of the radicicoles becomes accelerated, as may be observed 
from Table XXII. 
Table XXII. 
-Molting records of radicicoles of the grape phylloxera, March to 
July, 1915, Walnut Greek, Calif. ' 
Indi- 
vid- 
ual 
No. 
Date egg 
hatched. 
Date of 
first 
molt. 
First 
instar. 
Date of 
second 
molt. 
Sec- 
ond 
instar. 
Date of 
third 
molt. 
Third 
instar. 
Date of 
fourth 
molt. 
Fourth 
instar. 
Total 
grow- 
ing 
period. 
Aver- 
age 
tem- 
pera- 
ture. 
Gen- 
era- 
tion. 
1 
2 
3 
Mar. 19 
...do.... 
...do.... 
Apr. 3 
Apr. 7 
Apr. 8 
Apr. 10 
May 27 
...do.... 
June 7 
June 27 
June 30 
Days. 
15 
19 
20 
22 
16 
16 
15 
11 
14 
17 
Apr. 10 
...do.... 
Apr. 13 
Apr. 17 
May 30 
...do.... 
June 10 
June 30 
July 2 
July 6 
Days. 
7 
3 
5 
7 
3 
3 
3 
3 
2 
3 
Apr. 16 
...do.... 
Apr. 18 
Apr. 21 
June 1 
...do.... 
June 13 
July 1 
July 4 
July 9 
Days. 
6 
6 
5 
4 
2 
2 
3 
1 
2 
3 
Apr. 22 
...do.... 
Apr. 23 
Apr. 25 
June 5 
...do.... 
June 16 
July 4 
July 9 
July 10 
Days. 
6 
6 
5 
4 
4 
4 
3 
3 
5 
1 
Days. 
34 
34 
35 
37 
25 
25 
24 
18 
23 
24 
°F. 
58.25 
58.25 
58.25 
58.25 
61 
61 
63 
65 
65 
65 
A. 
A. 
A. 
4 
5-'.... 
6 
7 
8 
9. 
...do.... 
May 11 
...do.... 
May 23 
June 16 
...do.... 
A. 
B. 
B. 
B. 
C. 
c. 
10.... 
...do.... 
July 3 
C. 
The individuals enumerated in Table XXII were reared under 
cellar conditions on equally succulent pieces of severed roots. Table 
XXII, both by itself and taken in conjunction with Tables XX and 
XXI, indicates the influence of temperature upon the development 
of the radicicole under equal food conditions. Under a temperature 
of 58.25° F. the period of growth averaged 35 days, under an aver- 
