272 
Journal of Agricultural Research 
Vol. XXVIII, No. 3 
Table I.— Dimensions in microns of the cephalopharyngeal skeleton and mouth 
hooks and the number and dimensions of part of the spinous armature of the 
first , second, and third larval instars of Hypoderma lineatum 
Instar 
Total 
length 
Expanse 
of mouth 
hook 
from 
tip to 
tip 
Length 
of ceph¬ 
alophar¬ 
yngeal 
skeleton 
and 
mouth 
hooks 
Num¬ 
ber of 
spines 
in 
group 
below 
mouth 
Diam¬ 
eter of 
eighth 
seg¬ 
ment 
Spines on ventral side of eighth segment 
First row 
Diameter 
at base 
Length 
Distance 
apart 
Num¬ 
ber 
in 
row 
Mini¬ 
mum 
Maxi¬ 
mum 
Mini¬ 
mum 
Maxi¬ 
mum 
Mini¬ 
mum 
Mini¬ 
mum 
1. 
419 
14.1 
115.6 
36 
114 
2.6 
3.4 
5.4 
6.3 
1.5 
3.3 
23.0 
2. 
2,312 
15.3 
117.1 
55 
731 
2.7 
3.5 
4.5 
9.8 
17.0 
25.6 
30.6 
3. 
13,422 
16.6 
125.5 
57 
3,462 
0.0 
Spines on ventral side of eighth segment 
Second row 
Third row 
Instar 
Diameter 
at base 
Length 
Distance 
apart 
Num¬ 
ber 
Diameter 
at base 
Length 
Distance 
apart 
Num¬ 
ber 
Mini¬ 
mum 
Maxi¬ 
mum 
Mini¬ 
mum 
Maxi¬ 
mum 
Mini¬ 
mum 
Maxi¬ 
mum 
in 
row 
Mini¬ 
mum 
Maxi¬ 
mum 
Mini¬ 
mum 
Maxi¬ 
mum 
Mini¬ 
mum 
Maxi¬ 
mum 
in 
row 
1. 
2.0 
2.9 
4.1 
5.5 
1.6 
3.9 
24.0 
1.8 
2.5 
2.9 
3.8 
3.0 
5.0 
22.3 
2. 
1.9 
2.8 
4.2 
8.2 
12.8 
31.6 
29.6 
1.6 
2.4 
4.5 
6.1 
10.9 
36.3 
30.6 
3. 
0.0 
0.0 
Spines on ventral side of eighth segment 
Anal segment 
Fourth, 
fifth, sixth and seventh rows 
Spines 
Length of 
Instar 
Diameter 
at base 
Length 
Distance 
apart 
Num¬ 
ber 
Diameter 
at base 
Length 
triangular 
spines on 
border of 
Diameter 
of pos¬ 
terior 
Mini¬ 
mum 
Maxi¬ 
mum 
Mini¬ 
mum 
Maxi¬ 
mum 
Mini¬ 
mum 
Maxi¬ 
mum 
in 
row 
Mini¬ 
mum 
Maxi¬ 
mum 
Mini¬ 
mum 
Maxi¬ 
mum 
posterior 
spiracles 
spiracles 
1. 
1.4 
2.9 
2.1 
4.7 
2.3 
6.6 
21-23 
2.4 
3.3 
4.3 
7.3 
12.1 
5.9 
2. 
3. 
1.4 
2.3 
3.7 
6.1 
11.6 
37.6 
29-31 
0 
2.7 
6.5 
7.6 
22.5 
4.2 
5.7 
10.5 
17.4 
10.3 
5.6 
9.4 
14.1 
The figures in the table reveal marked differences, particularly in the number 
and the dimensions of the spines of the stages in question. The cephalopharyngeal 
skeleton and mouth hooks increase in size, as shown by the figures in the table, 
and the posterior part of the skeleton changes its shape from one stage to the 
next, spreading out with each molt. Although these differences are not great, 
there is shown, nevertheless, constant increase in the size of these highly chit- 
inized organs. 
If the spines in the group below the mouth are next examined a wide differ¬ 
ence in number is found, especially between those of the first and second stages. 
Since the figures in the table represent averages for each stage, this difference 
could not be accounted for by individual variation. Among the spines on the 
ventral side of the eighth segment just as marked a difference in dimensions and 
number is found. With only one exception, that of the first row, the maximum 
diameter of the spines at the base is larger in the first than in the second stage. 
Although the diameter of these spines has decreased slightly from the first to 
the second stage, the length, without exception, has nearly doubled and the 
number of spines has likewise increased from 23 to 30.6 for the first row and 
similarly for all the other rows. Finally, when the spinous armature of the ter¬ 
minal segment is considered, the most striking differences are found. The spines 
