﻿80 NINTH ANNUAL REPORT 



tain region to the west of that province, it is more than probable that 

 the principal source of the 1876 invasion was Montana and the Saskat' 

 chawan and Swan River countries. The question as to how far the 

 northwest breeding grounds are recruited by the insects which hatch 

 in the more fertile country which I have designated as outside the 

 species' natural habitat, is a most interesting one; for if thus recruited 

 there is all the greater incentive for us to exterminate the young 

 insects which hatch with us. All such questions can only be settled 

 by a thorough study of the subject by a properly constituted com- 

 mission, charged by Congress with the work. 



EASTERN LINE REACHED. 



A study of the eastern limit of the invasion of 1876, compared 

 with that of 1874, shows that it is peculiar in reaching farther east in 

 Minnesota and Iowa, and farther south and east in Texas. The limit- 

 line — extending from Clay county, Minnesota; bulging toward St. 

 Paul, reaching southwardly to the center of Iowa; thence westwardly 

 receding to Lawrence, Kansas, and bulging again to Southwest Mis- 

 souri — is more irregular between the 36th and 46th parallels than it 

 was in 1874. On an average, however, it does not extend east of 

 the 94th meridian. 



RATE AT WHICH THE INSECTS SPREAD. 



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 Da- 

 kota and Nebraska, W. Minnesota, N. W. Iowa, N. W. Kansas, and N. 

 E. Colorado — they appeared almost simultaneously, or during the last 

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

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

 higher rate by the N. W. winds from Montana and British America. 

 After that time the extension S. was tolerably rapid, but the exten- 

 sion E. was more and more slow. They occupied nearly a month 

 reaching from N. W. Iowa to the S. W. limit in the same State, and 

 their eastward progress on the confines of the limit line already indi- 

 cated was still more gradual as they went South. All of which indi- 

 cates that they fly most powerfully when leaving the higher altitudes 

 of the N. W., and most persistently during the first week or so after 

 becoming fledged, while the females are not yet prompted to descend 

 for oviposition. This is also the period when they are passing over 

 the vast plains and the sparsely settled and uncultivated portion of 

 the country, in which there is, perhaps, least inducement for the raven- 

 ous host to halt. 



