THE ROCKY MOUNTAIN LOCUST A SUB-BOREAL INSECT, 131 



and especially the raising of grain, are those where the locust breeds in 

 greatest profusion, and where the climatic conditions are all favorable to 

 its propagation. 



On the other hand, it seems obvious that the more that is done bj^ 

 farmers in the future in destroying the young locusts within this perma- 

 nent area, the more likely will their combined efforts, carried on from 

 year to year, tend to the ultimate destruction of the locusts. And it 

 follows that the more extensively this region is settled, the more will 

 the locusts diminish in numbers. It is, then, only a question of time, 

 and of immediate effort on the part of the present generation of farmers. 



Another fact is established and proved by study of the distribution of 

 the breeding grounds of this species of locust. While Caloptenus femur- 

 rul)rum^ the common red-legged locust, and C. atlanis, the lesser rod-leg- 

 ged locust, have an immense natural range, breeding annually in abund- 

 ance from Middle Florida to near the Arctic Circle or northern limit of 

 forests, on the east and westward ranging as far south, approximately, as 

 the 40th parallel in Utah and California, C. spretus is permanently re- 

 stricted to an area north of the 40th parallel, only extending southward 

 along the Eocky Mountain Eange, or periodically visiting the Great Basin 

 south of this parallel. On the other hand, its more extensive and con- 

 tinuous breeding grounds lie north of the 42d parallel and on the ele- 

 vated plateau of the Eocky Mountains above an altitude of 2,500-3,000 

 feet. It breeds in destructive numbers in the Eocky Mountains south 

 of this parallel, between 5,000 and 8,000 feet elevation. North of the 

 53d parallel it does not probably extend, and it is not so boreal and even 

 subarctic an insect as the two previously named species, both of which 

 occur on the summit of Mount Washington, in New Hampshire, while 

 C. atlanis ranges as far north, on the Pacltic Coast, as the Yukon Elver. 



It would be safe to regard Caloptenus spretus or the Eocky Mountain 

 locust as a north subtemperate or sub-boreal insect, viz, an inhabitant 

 of what is called by naturalists the north subtemperate zone of life, situ- 

 ated between the annual isotherm als of 50° and 36°.^" In vertical distri- 

 bution it may be said to breed from an altitude of about 2,000 feet up as 

 far as 10,000 feet, or near the timber line in the Eocky Mountains, though 

 few probably breed in great numbers above an altitude of 8,000 feet. 

 The elevated plains of Colorado, Northern Utah, Idaho, and W^yomiug, 

 where it breeds so abundantly, correspond in a general way to the rather 

 lower plains of Northern Montana and British America. The Eockj- 

 Mountain locust, then, like the antelope, the Eocky Mountain sheep, the 

 prairie dog, the bison (as now restricted in its range), pouched mar- 

 mot, and a number of peculiar birds, reptiles, insects, Crustacea, and 

 many trees and plants, is a form purely characteristic of the central 



20 The boreal zone, in jSTorth America, comprises the Canadian and American subarctic zones. "^Vhile 

 C ■ femur-ruhrum (and probably C. atlanis) ranges from the subarctic zone, through the lorcal, Canadian, 

 Alleghanian, Carolinian, and Floridan belts, C. spretics ranges permanently throu'gh what corresponds in 

 the central region of the continent to the Alleghanian or subtemperate and Carolinian or tcaiiierate 

 zones or divisions. There are some features inthe animal life of this central province which strikingly 

 recall the forms characteristic of the steppes and plateaus of Northeastern Asia. 



