THOMAS ON OETHOPTERA OF DAKOTA AND MONTANA. 499 



Although the resulting brood generally proves more destructive in 

 the mountain regions than the incoming storm, yet this does not appear 

 so far to have been the case in the Mississippi Valley ; and as a pre- 

 ventive or remedy for the original hordes dispenses with the necessity 

 of battling with their progeny, it is against these the general govern- 

 ment should direct its efforts in an earnest and determined manner. 



From what is known of the habits of this species we may be assured 

 that it will never become a permanent resident of the Mississippi Val- 

 ley, as its sudden transfer from the dry and rarified air of the elevated 

 mountain regions to the heavy and moist atmosphere of the States 

 requires too rapid a change in its nature for it to undergo. But, sup- 

 posing it should become habituated to this region and overcome all 

 climatic difSculties, it is very probable, in fact I might say almost cer- 

 tain, that it would lose its migratory disposition, and if but a variety of 

 C. femur-ruhrum, as I strongly suspect, would in all probability revert 

 to that form. 



The origin of the migratory habit of this species is an interesting 

 question, and, I am inclined to think, is directly connected with the^ 

 origin of the treeless plains of those western regions. If, as I have 

 intimated, it is a variety of C. femur-ruhrum, it is highly probable the 

 latter appeared first in the older districts of the Atlantic area in its 

 present or some earlier form, and gradually extended west, and, as is 

 usual with the group to which it belongs, as it ascended to the colder 

 regions of the Eocky Mountain Range, would have assumed the short- 

 winged form, unless prevented by some compensating cause. The re- 

 peated burnings of the prairies may have caused frequent removals, and 

 thus have given origin to its longer wings and migrating disposition. 

 I am aware the question may be asked, Why did not the same thing 

 occur with other species *? But if the reader will carefully examine the 

 list and localities of the United States Acridil, he will find but few spe- 

 cies which belong to both the eastern and western regions ; the belt 

 which once formed the water-line north and south through the conti- 

 nent forms a more distinct line between Acridian districts than even 

 the Eocky Mountain Eange, as I have shown in a former j)aper. But 

 this is a question requiring a more thorough investigation than I can 

 give it in this note, even had I the data necessary and felt able to do 

 so with my limited geological knowledge. I therefore simply throw 

 out the thought, to call the attention of others to the subject. 



There is another fact presented this season in regard to this group of 

 the Caloptenij to which I wish for a moment to call attention. 



As shown in my Synopsis, and as confirmed by other entomolo- 

 gists, the chief difference between the spretus and femur-rubrum is the 

 notch in the last abdominal segment of the male in the former and its 

 absence in the male of the latter species, and the longer wings of the 

 former. 



Heretofore, the femur-ruhrum^ as thus marked, has always been our 



