156 EEPORT UNITED STATES ENTOMOLOGICAL COMMISSION. 



abundance, as in 1874 and 1876, and when other circumstances favor, 

 they do not jQy west and southwest, but there is a general movement 

 eastward beyond the plains to the border States. On the other hand, 

 Montana is liable to be visited by "return flights" of locusts which 

 have hatched in the border States. Thus, when the border States are 

 invaded, it would appear as if the Northwest, namely, Montana, North- 

 ern Wyoming, and British America, is partially depopulated, and it 

 would seem as if this general exodusfrom the permanent breeding-ground 

 was an exceptional phenomenon. For example, in 1877, while great 

 numbers hatched in Colorado, Utah, Idaho, and the border States, Cen- 

 tral Montana and British America north of it were almost entirely free 

 from locusts, and this was most probably due to the exodus of locusts 

 from this region in 1876. 



Whether the swarms which entered Central Montana, in the year 1877 

 originated in the Judith and Yellowstone Basins, or flew from the bor- 

 der States, we do not know, but we are disposed to think that they were 

 of local origin. Still, swarms flying from the southeast were observed 

 July 20, on the Little Big Horn, by Lieutenant Carpenter. 



The fact has been established, we think, that the swarms which usu- 

 ally invade Central Montana originate in the region east of the Belt 

 Mountains and in British America. They lay eggs in the Sun River 

 and Gallatin Valleys, for example, and the following year their progeny, 

 as a rule, infest Eastern Idaho and Northern Utah, and the year fol- 

 lowing perhaps Central and Southern Utah. At any rate, as shown on 

 our map, the main source of supply of the locusts which infest Utah, 

 and the region about Franklin, Idaho, is Central Montana ; Central 

 Montana receiving its locusts primarily from Eastern Montana and 

 Bri ish America. 



It is impossible to lay down general laws, as the movements of the 

 locusts are as variable and uncertain as the direction of the winds and 

 the condition of the weather in that mountainous country. We have 

 only attempted to give the general facts of the migrations, and find it 

 impossible to lay down absolute laws in the present state of our knowl- 

 edge*. 



It would also appear that the region east of the Belt Eange, and possi- 

 bly the Big Horn Mountains, may form a dividing line, east of which 

 the locusts fly east, and west of which they fly west ; certainly the Belt 

 Range is not a barrier to their westward flight, although the Bitter Root 

 Range appears to be, deflecting the swarms southward into Eastern and 

 Southern Idaho and Northern Utah. 



Wyo7ning. — This Territory is so thinly settled, and consequently obser- 

 vations on the migrations of the locust so scanty and incomplete, that 

 it is'difiicult to draw any general conclusions as to the movements of 

 the swarms that breed in such great numbers in the larger river valleys. 

 The most reliable observations tend to show that the swarms fly in a 

 general southeast, sometimes east course. By reference to the arrows 



