90 



The specimens referable to variety umhrosa have a more olivaceous 

 cast and are generally lighter in color. The stigmal spot is nearl}^ 

 always very 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 ochracea. 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 alwa3^s obsolete. 



More 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 XXXV. — Comparative sizes of hollworm moths. 



Sex. 



Wing expanse. 



Length. 



Largest. 



Smallest. 



Average. 



Largest. 



Smallest. 



Average. 



Male 



Millimeters. 

 47 

 46 



Millimeters. 

 30 

 34 



Millimeters. 

 40.42 

 40.86 



Millimeters. 

 21.9 

 20. 5 



Millimeters. 

 13 

 14.5 



Millimeters. 

 18.5 

 17.89 



Female : . . 



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, probabl}^ on account of their longer and 

 more slender abdomen. 



VARIATION AND POSSIBLE CAUSES. 



In spite of the very apparent A^ariation of the moths, the causes 

 which govern this variation are very 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 larvae raised on corn and on cotton under the same climatic 

 conditions. 



On several occasions it was noticed that moths emerging from pupae 

 which had been placed in cold storage and kept at a low temperature 

 for several days proved to be much darker than an}^ specimens we had 

 seen elsewhere. On the other hand, no especiall}^ dark individuals 

 are to be noticed among the moths of the spring brood whose pupa? 

 have been subjected to the graduall}^ lowering temperature of the win- 

 ter months. This shows plainly that the moths may be influenced b}^ 

 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. , 



