Habits, and Power for Injury. 



95 



One noticeable feature of the invasions is the greater 

 rapidity with which the insects spread in the earlier part 

 of the season, while in fullest vigor, and the reduction in 

 the average rate of progress the farther east and south 

 they extend. The length of their stay depends much upon 

 circumstances. Early in the summer, when they first 

 begin to pour down on the more fertile country, they sel- 

 dom remain more than two or three days ; whereas, later 

 in the season, they stay much longer. In speaking of the 

 advent and departure of these insects, I use relative lan- 

 guage only. The first comers, when — after having devoured 

 everything palatable — they take wing away, almost always 

 leave a scattering rear-guard behind, and are generally fol- 

 lowed by new swarms ; and a country once visited presents 

 for weeks the spectacle of the insects gradually rising in 

 the air between the hours of 9 or 10 a. m. and 3 p. m., and 

 being carried away by the wind, while others are constantly 

 dropping. 



KATE AT WHICH THE INVADING SWARMS SPREAD. 



This may be illustrated by the history of the 1876 in- 

 vasion. Leaving Montana about the middle of July, the 

 insects reached far into Texas by the end of September, 

 thus extending about 1,500 miles in 75 days, or an average 

 of about 20 miles per day. But over a large part of this 

 territory, viz., portions of Wyoming, most of Dakota and 

 Nebraska, Western Minnesota, Northwestern Iowa, North- 

 western Kansas, and Northeastern Colorado — they appeared 

 almost simultaneously, or during the last few days of July 

 and the first few days of August; and this, I think, indicates 

 that they were at that time swept down at a very much 

 higher rate by the northwesterly winds from Montana and 

 British America. After that time the extension south was 

 tolerably rapid, but the extension east was more and more 



