12 
BULLETIN 329, IT. S. DEPARTMENT OF AGRICULTURE. 
closely arranged cells (PI. Ill, fig. 2). The interior of an egg con- 
taining an embryo three- fourths developed has a peculiar banded 
appearance (PI. Ill, figs. 3, 4), but as the time of hatching ap- 
proaches the most noticeable features are the cephalic eye spots on 
the embryonic larva, which become more and more distinct. (PL III, 
fig. 5.) Detailed studies on the embryology of Simulium have been 
made by Mecznikow, 1 Kolliker, 1 and Graber. 1 
The average period of incubation, from June to October, of the 
species studied by the writer at Spartanburg, S. C, was about 8.8 
days; the minimum period, 7 days; and the maxium, 12 days. The 
following table will show the normal variation in that region from 
June to October: 
Table shotting period of incubation of eggs of Simulium. a 
T.ot 
Species. 
Date of 
ovipo- 
sition. 
Number of days of incubation. 
No. 
1 
2 
3 
4 
5 
6 
7 
8 
9 
10 
11 
12 
13 
14 
June 12 
June 21 
July 2d 
Sept. 6 
Sept. 11 
Sept. 16 
a 
a 
a 
a 
a 
a 
a 
a 
a 
a 
a 
a 
a 
a 
a 
a 
a 
a 
a 
a 
b 
a 
b 
b 
b 
b 
a 
b 
b 
a 
a 
a 
a 
b 
a 
b 
b 
b 
b 
b 
b 
b 
c 
c 
b 
b 
b 
b 
b 
a 
c 
b 
c 
c 
c 
c 
b 
c 
c 
c 
b 
c 
b 
c 
b 
c 
c 
c 
c 
c 
c 
c 
d 
d 
c 
c 
c 
c 
c 
b 
d 
c 
d 
d 
d 
d 
c 
d 
d 
d 
c 
c 
c 
d 
b 
h 
d 
h 
b. 
h 
b 
d 
b 
b 
d 
d 
d 
c 
b 
c 
d 
h 
d 
b 
b 
2 
3 
do 
b 
b 
b 
b 
4 
S. bracteatum 
T 
do 
c 
c 
d 
b 
fi 
7 
do 
b 
8 
do 
...do 
9 
do 
...do 
10 
do 
...do 
11 
...do 
I 9 
do 
do 
do 
do 
...do 
...do 
...do 
...do 
b 
b 
b 
b 
13 
b 
b 
b 
14 
b 
b 
15 
b 
1 
a Letters indicate stage of incubation, shown on Plate I, as follows: a, corresponds to fig. 1; b, corresponds 
to fig. 2; c, corresponds to figs. 3, 4; d, corresponds to fig. 5; b, date of batcbing. 
The eggs seem to hatch equally well in either still or running water. 
The percentage of eggs that hatch is very high when they are placed 
so that the water reaches them, the average sterility of a series of 
lots kept under observation in normal conditions being from 4 to 5 
per cent. The approximate percentage of egg masses, however, that 
is deposited under such favorable conditions is certainly relatively 
small as compared with the total number deposited, and the location 
of the egg mass seems to be the dominant factor in determining the 
successful hatching of the larva and the consequent abundance of 
the insect. 
THE LARVAL STAGE. 
The larvse of the known species of Simulium, as will be discussed 
at length farther on, are invariably aquatic in their habits and re- 
1 See Bibliography, p. 35, 
