624 REPORT UNITED STATES GEOLOGICAL SURVEY. 



of going northward, as is usual with the spring broods hatched in the Missouri River 

 Valley. No northward or return flight was noticed over the Big Horn region during 

 the season of 1876. 



About the last of July, 1876, the great flights from the northwest swarmed over the 

 country along the eastern base of the Big Horn Mountains. They came in such num- 

 bers as to create a hazy atmosphere which was at first supposed to be due to prairie 

 fires. During severals days they covered the earth and obscured the sun. Their line 

 of flight was from the west of north, and in general appeared to conform to the contour 

 of the mountain-range, following it to the southward and eastward. For some time 

 after reaching the ground they seemed bewildered and inactive, but gradually recovered 

 and commenced eating voraciously and pairing off". I did not, however, observe them 

 lay eggs here. After remaining about a week they nearly all left one afternoon just as 

 a stiff breeze was springing up from the northwest, their flight being stiU to the south- 

 east. Of those which remained behind, about one-fOurth appeared disabled for vigor- 

 ous flight from the presence of the eggs of the parasitic mites which destroy so many ; 

 the little red oblong mites which were firmly attached to the under side of the wings, 

 impeding them greatly and causing their ultimate destruction. Many other individuals 

 had a sickly appearance, though I could discover nothing unusual affecting then except- 

 ing a general paleness of the body and wings and extreme weakness. These swarms 

 appeared in Western Nebraska about the middle of August. 



Upon leaving the Big Horn Mountain I passed in a northeasterly direction over the 

 region drained by the Rose Bud, Tongue, Powder, and Yellowstone Rivers, and every- 

 where found evidence, in the condition of vegetation and the large quantity of " frass " 

 on the ground, that a flight had also been here. I believe this to have been part of 

 the great flight observed at the base of the Big Horn Mountains, and it consequently 

 must have covered, at the same time, about 12,000 square miles of territory. 



It would appear as though the great swarms, which are so destructive to eastern 

 vegetation, follow in their eastward flight the general trend of our western mountains. 

 Starting, as we suppose, from the Great Plains at the head of the Saskatchewan River, 

 they would follow it down to the spurs of the Rocky Mountain chain, which curve to 

 the southeastward and ofter a continuous area of vegetation to sustain them in their 

 journey. The Big Horn Mountains next come in view, and by their contour tend to 

 place them well to the south by the time they reach the southern end. 



The next objective point in their flight would be the Black Hills, which are densely 

 timbered, and would naturally attract them from a distance. After leaving this latter 

 region there are no prominent elevations to guide them in their flight, and they con- 

 sequently follow the drainage of the Missouri Valley, stopping when the bright green 

 fields of our farming communities are reached and suitable food obtained. The short 

 prairie grasses at the season of their migratory flight have lost their freshness and 

 begun to turn yellow. The prairies are consequently not attractive to them, and they 

 only halt briefly for rest. But when the tall, luxuriant vegetation of the east is 

 reached, they instinctively realize that they have arrived in a land of plenty, and 

 accordingly leave their eggs where their young will find abundance of food, upon 

 hatching out. 



In the southward flight of these great swarms, I believe that they never extend as 

 far as Laramie Plains. The valley of the Upper North Platte appears to be the southern 

 limit of their migration, although they cross the Platte farther to the eastward and 

 overrun Eastern Colorado and Kansas. The Upper North Platte Valley is a region of 

 very high winds and perhaps unsuited for this reason as a highway of travel ; while 

 the line of flight chosen by them is through a country unusually free from atmospheric 

 disturbances. 



The grasshoppers which visit that part of Colorado extending for some distance east- 

 ward of the mountains, I believe, take their flight from Utah, and travel nearly due 

 east over the vast intervening mountain-ranges. The members of Professor Hayden's 

 Geological Survey of Colorado observed them in 1873, on the summits of all the high- 

 est peaks, in yast numbers. I have never heard of a flight crossing the Union Pacific 

 Railroad near Cheyenne, Wyo., and moving due south into Colorado along the mount- 

 ain-range which here runs north and south. And this would have been observed 

 if they entered Northern Colorado from the north instead of from the west. The main 

 range of Colorado, from its great altitude, offers a formidable barrier to their eastward 

 progress, causing myriads to perish from the cold which pervades these elevated re- 

 gious. 



If thecountry between theBig Horn Mountains and the Black Hills and the Upper Mis- 

 souri and North Platte Rivers were a thickly-settled farming region, the greatswarms 

 on their eastward journey would stop here, and never reach the Lower Missouri Valley. 

 The westward progress of civilization must ever decrease the amount of damage done 

 to eastern agriculture ; and finally when the entire West shall be settled, the cultivated 

 fields will be extended over such a wide area that the swarms will be proportionately 

 arrested and scattered, and the destruction of crops in any particular State be incon- 

 siderable. 



