148 
Journal of Agricultural Research 
Vol. XX X, No. 3 
Table III .—Range of incubation period 
of eggs of Phlyctaenia rubigalis during 
six generations 
Generation 
Total 
num¬ 
ber of 
eggs 
depos¬ 
ited 
Incubation period 
Mini¬ 
mum 
Maxi¬ 
mum 
Aver¬ 
age 
Second___ 
142 
313 
3,400 
(«) 
570 
142 
Days 
5 
8 
5 
5 
4 
4 
Days 
9 
10 
10 
6 
7 
6 
Days 
7.0 
9.0 
7.5 
5.5 
5.5 
5.0 
Third_ 
Fourth ... 
Fifth__ 
Eighth_ 
Ninth.... 
Average. 
6. 58 
<* Few. 
EGG STAGE 
Incubation 
Consulting Table III, it will be seen 
that the incubation period varies, 
being from 4 to 10 days in length and 
averaging 6.5 days. It was also found 
that all the eggs do not necessarily 
hatch at the same time. From a total 
of 50 eggs deposited on a marguerite 
plant March 10, 17 hatched on March 
19, or 9 days later, while the remaining 
33 did not hatch until the following 
day. Another lot of eggs deposited 
on the same date on other plants began 
hatching on March 18, or 8 days after 
deposition. 
Development 
The time required for the embryonic 
development of the young larvae was 
observed to vary from 4 to 10 days, 
depending, no doubt, on the tem¬ 
perature and other factors. It was 
difficult to observe the earlier changes 
without the aid of a microscope, be¬ 
cause of the reticulated structure of the 
shell, which is iridescent. However, 
several days after deposition the shell 
becomes more transparent, and the 
outer margin more distinct. Shortly 
before hatching the eggs appear more 
plump, and segmentation can be 
recognized. 
Newly deposited eggs appear to have 
a vague granular, cellular structure. 
The outline of the cells soon becomes 
more distinct, and definite layers of 
them are grouped in the periphery of 
the egg. Invagination takes place at 
one side, and these cells appear to lose 
their identity and break down, evi¬ 
dently for the purpose of forming the 
various organs and internal structures 
of the body. The first conspicuous 
change observed was the presence of 
orange colored spots at one end of the 
egg, which soon developed into red¬ 
dish-brown mouth parts. Even before 
the outlines of the head were apparent , 
the mandibles could be seen, opening 
and closing. The eye spots can be 
seen gradually growing darker, and the 
legs, prolegs, bristles, and dorsal vessel 
can be distinguished before segmenta¬ 
tion is completed. At about the same 
time movement by the larva appears to 
rupture the walls of the yolk cells sur¬ 
rounding the body, and forces the 
contents toward the anterior end of the 
body, where it is swallowed. After 
intermittent periods of activity and 
rest the larva is then ready to hatch. 
Hatching 
The larva, when ready to hatch, is 
in a U-shaped position and occupies 
almost the entire space within the 
egg. Contact with the eggshell is 
loosened by backward and forward 
movements of the body, and the larva 
retracts its head before forcing the 
mouth parts against the shell. The 
mandibles operate with a pincerlike 
action in tearing the hole through 
which it later escapes. As soon as the 
jaws break through, the head and first 
two segments are thrust out, and the 
larva employs its posterior legs and 
setae to good advantage in crawling 
free from the collapsed shell. From 
the time when the mandibles first pene¬ 
trated the shell until the larva crawled 
away, hatching was observed to re¬ 
quire from 4 to 46 minutes. 
Observations on hatching from a 
cluster of eggs show that the larvae 
may begin emerging one after another 
but that sometimes two or three may 
emerge at the same time. In a 
group of five larvae which hatched 
March 25, two larvae becoming active 
at the same time showed a difference 
of only 20 seconds in time of hatching. 
The other three hatched at intervals 
of 15 and 30 minutes later. On 
April 14, hatching began in a group 
of seven eggs at 1.54 p. m. and was 
completed two hours later. The larvae 
within two eggs, which were partially 
overlapping, became active at the 
same time, but interfered to such an 
extent that one required two minutes 
longer than the other to get free from 
the shell. The observations in Table 
IV show the intervals of time required 
between the breaking of the shell and 
the crawling away of the freed larva. 
