THE GRAPE PHYLLOXERA IN CALIFORNIA. 33 
The rate of egg deposition is usually indicated by a sharp rise shortly 
after commencement, followed by a gradual decline. During the 
period of egg laying the adult feeds, and after the last egg is laid 
may live for as long as three weeks. 
Incubation naturally is influenced by temperature, and the dura- 
tion of the incubation period may vary from five days in July to 
over a month in December. Very few eggs are laid in December, but 
in March and April, when many eggs are deposited, the maximum 
period of incubation is 27 days. 
The larvae mature in midsummer in about 15 days, and in April 
and November in about 34 days, and the hibernant generation de- 
velops in about 180 days. The winged forms mature more slowly 
than do the wingless individuals, since the fourth or nymphal instar 
is noticeably extended beyond that of the corresponding wingless 
stage. 
In the late fall a few individuals intermediate in structure be- 
tween the nymphs and radicicoles are found. These are called 
" nymphicals " or intermediates and, so far as is known, they de- 
posit the same type of eggs as the radicicoles, although they are 
not prolific. From egg deposition to the molting of the final skin, 
the period covered by the sexes, which develop from eggs of two 
sizes laid by the winged forms, was about 12 days in confinement. 
All stages of the phylloxera molt four times, and the first instar 
is always the longest (the adult instar excepted). 
HIBERNATION. 
The phenomenon of hibernation. — Throughout autumn and early 
winter an ever-increasing percentage of newly hatched radicicole lar- 
vae, instead of increasing in size and maturing normally, remain 
as very small brown phylloxeras (PI. IX, d, p. 64). As winter pro- 
gresses, the mature individuals die, leaving only the small brown 
larvae and a few unhatched eggs. As soon as these late eggs hatch, 
the larvae settle down, becoming brown like the others. These small 
larvae are the hibernants, and as such they remain throughout the 
dormant period. Occasionally phylloxeras that have passed one or 
two molts hibernate. This type is quite unusual, and probably con- 
sists of individuals that have reached a certain stage of development 
and are unable, through lack of nourishment, to mature, most of them 
dying before spring. 
Hibernant larvae occur on all kinds of vines — on viniferae and 
on American varieties and hybrids. While this form of phylloxera 
occurs more or less sparingly on American resistant vines (Vitis 
riparia. V. rupestris, V. berlandieri, etc.) and on some American 
1900°— 21 3 
