RATE AT WHICH LOCUST SWARMS MOVE. 215 



ways, and they shall Dot break their ranks. * * * They shall 

 run to and fro in the city ; they shall run upon the wall ; they shall 

 climb up upon the' houses j they shall enter in at the windows like a 

 thief.'" 



Persons in the East have often smiled incredulously at our statements 

 that the locusts often impeded the trains on the western railroads. Yet 

 such was by no means an infrequent occurrence in 1874: and 1875 — the 

 insects passing over the track, or basking thereon so numerously that 

 the oil from their crushed bodies reduced the traction so as to actually 

 stop the train, especially on an up-grade. 



While the destruction of crops by the winged insects is often sudden 

 and complete, the unfledged insects still more effectually, though more 

 slowly, denude a country of vegetation, sometimes rendering the ground 

 as bare and desolate in midsummer as it is in the Mississippi Valley in 

 midwinter. The little creatures are often so thick, soon after hatching, 

 that they blacken everything, and their hopping, as one passes through 

 a field or piece of prairie, gives the impression, at a short distance, as 

 suggested by Mr. Whitman, of heat flickering in the air. 



The migratory habit and great destructive power belong essentially 

 to the Rocky Mountain locust. 



As will appear from the two concluding chapters of this work, there 

 are three or four very destructive and migratory species of locusts in 

 Europe and Asia. There are also several other species which some- 

 times become very destructive, and still more rarely migrate from place 

 to place in this country. Yet the Eocky Mountain locust is essentially 

 the migratory and destructive species of North America, as none other 

 compares with it in the vastness of its movements or the injury which 

 it inflicts. 



KATE AT WHICn LOCUST SWARMS MOVE. 



The rate of migration of the winged insects will depend entirely on 

 circumstances. The history of the past four years shows conclusively 

 that the rate of progress of invading swarms from the permanent breed. 

 ing-places will average about 20 miles a day. It is, however, exceed, 

 ingly irregular, and greatly dependent on the velocity of the wind. 

 Bad weather may impede, or adverse winds divert flight. 



" One noticeable feature of the invasions is the greater rapidity with 

 which the insects spread in the earlier part of the season, while in full- 

 est 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 seldom 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 

 language only. The first comers, when — after having devoured every- 

 thing palatable — they take wing away, almost always leave a scattering 

 rear-guard behind, ar.d are generally followed by new swarms ; and a 



