April, ’20] 
GLENN: OYSTER-SHELL SCALE 
175 
is nearly gray and lighter in color than the exuvia. It is single-brooded 
and hatches at Urbana, Ill., during the third or fourth week in May. 
The yellowish-brown form has been taken on cornus, lilac, soft 
maple and Rosa rugosa at Urbana. This form resembles the brown 
form very closely, but the posterior half of the scale is lighter in color 
than the anterior part and is in most cases distinctly yellowish. This 
characteristic cannot always be depended upon to distinguish it from 
the brown form, but in connection with one of the microscopic char¬ 
acteristics of the two forms they can be quite readily distinguished. 
It is double-brooded, and is usually heavily parasitized. 
Many specimens of each of these forms have been examined under 
the microscope and with the exception of the number of circumgenital 
pores they seem to be identical. Specimens were submitted to Dr. 
A. D. MacGillivray. He made a careful comparison of the structural 
characteristics which up to the present time have been used to deter¬ 
mine the species in this genus and reported that he could find no 
difference. 
It is interesting to note, however, that there is a striking and quite 
constant difference in the average number of circumgenital pores in 
these forms, as the following table will show: 
Circumgenital Pores 
Form of 
Num¬ 
ber 
Host plant 
Posterior lateral 
groups 
Anterior lateral 
groups 
Median group 
Total 
Aver- 
scale 
exam¬ 
ined 
Max. 
Min. 
Aver. 
Max. 
Min. 
Aver. 
Max. 
Min. 
Aver. 
age 
Brown 
30 
Apple 
21 
8 
12.8 
21 
9 
16.8 
12 
8 
9.5 
68.7 
30 
17 
8 
13.9 
23 
12 
19. 
14 
9 
12. 
77.8 
30 
y y 
17 
8 
13.5 
23 
13 
17.7 
17 
7 
11.5 
73.9 
30 
9f 
19 
8 
13.8 
23 
13 
16.8 
16 
6 
10.5 
71.7 
Average 
120 
13.5 
17.6 
10.9 
73.1 
Yellow- 
ish-brown 
36 
Cornus 
25 
13 
17.2 
24 
14 
19. 
17 
8 
12. 
84.4 
23 
y y 
21 
12 
16.1 
23 
11 
18.6 
16 
9 
12. 
81.4 
23 
Per. Lilac 
22 
15 
16.45 
23 
10 
19.1 
21 
8 
[13. 
84.2 
24 
Rosa Rugosa 
24 
13 
16.7 
24 
13 
18.8 
18 
10 
13.8 
' 85.6 
Average 
106 
16.5 
18.8 
12.6 
, 83.2 
Grayish- 
brown or 
banded 
27 
Lilac 
27 
16 
21. 
30 
15 
23.2 
21 
11 
14.4 
102.8 
20 
y y 
28 
17 
21.2 
26 
13 
21.6 
14 
9 
12.3 
97.9 
32 
y y 
27 
15 
19. 
28 
15 
21.5 
22 
8 
13.2 
94.2 
30 
Poplar 
32 
14 
21. 
29 
17 
22.7 
16 
10 
13.9 
101.3 
32 
Ash 
29 
17 
22.6 
29 
18 
24.7 
19 
12 
14.3 
108.9 
27 
Pus. Willow 
27 
17 
21.5 
28 
18 
23.3 
11 
11 
14.3 
103.9 
11 
y y yy 
30 
22 
24.3 
30 
17 
24.5 
17 
10 
15.1 
112.7 
27 
Rosa Rugosa 
31 
16 
23.5 
30 
17 
23.7 
20 
10 
15. 
109.4 
26 
Weep. Wil. 
29 
16 
23.2 
30 
21 
25. 
18 
9 
14.4 
110.4 
Average 
232 
21.9 
23.2 
13.9 
104.1 
There is a considerable variation in the number of circumgenital 
pores in individuals of the same group, so much so that it would be 
2 
