90 
The specimens referable to variety umhrosa have a more olivaceous 
cast and are generally lighter in color. The stigmal spot is nearly 
always A^er}^ plainly marked and the cross-band of the front wings 
more or less obsolete. The black apical band of the hind wings is 
not so pronounced as in oclvracea. Beneath, the markings are about 
the same, except that the band on the front wing is weaker and the 
one on the hind wing nearly alwa^'S obsolete. 
^lore of the females fall in the ochracea group and more of the males 
under umhrosa. 
SIZE OF MOTHS. 
Measurements of a series of 100 moths gave the following sizes: 
Table XKXV. — Comparative sizes of boll worm moths. 
Sex. 
Wing expanse. 
Length. 1 
Largest. 
Smallest. 
Average. 
Largest. 
Smallest. 
Average. 
Male 
Millimeters. 
47 
46 
Millimeters. 
30 
34 
MiUimcters. 
40.42 
Millimeters. 
21.9 
Millimeters. 
13 
Millimeters. 
18 5 
Female 
40.86 
20.5 
14.5 
17.89 
From this it can be seen that the males are considerably more vari- 
able in size than the females. In wing expanse they average a little 
smaller, but in length greater, probably on account of their longer and 
more slender abdomen. 
VARIATION AND POSSIBLE CAUSES. 
In spite of the very apparent variation of the moths, the causes 
which govern this variation are veiy obscure. Riley" thought that 
those moths feeding on corn in the Western States were darker and 
more brightly colored than those of the cotton belt. In our experi- 
ence in breeding no constant difference was noticed between moths 
bred from larv^ raised on corn and on cotton under the sanie climatic 
conditions. - 
On several occasions it was noticed that moths emerging from pup» 
which had been placed in cold storage and kept at a low temperature 
for several days proved to be much darker than any specimens we had 
seen elsewhere. On the other hand, no especially dark individuals 
are to be noticed among the moths of the spring brood whose pupiv 
have been subjected to the gradually lowering temperature of the win- 
ter months. This shows plainly that the moths may be influenced by 
temperature, but it is evident that there are other factors concerned 
also. 
To ascertain whether there was any relation between the extremely 
variable color of the larvae and the color of the moths, the color of a 
"Fourth Report, p. 371. 
