f,i?Fe:cts of winds in dispe:rsai.. 47 



for the grovving season. This acceleration by the wind is especially note- 

 worthy when the wind happens to correspond in direction with a natural 

 highway, as in the Great Lakes-St. Lawrence Valley. In this region the 

 lines of distribution from 1866 to 1872 became wider apart, until in 1871, 

 1872, and 1873 great stretches of country were overrun each year. While 

 other factors helped to produce this result, such as the beetles being carried 

 on the waters of the lakes, the major part of the advance ground covered 

 each year was due to the aid given by the wind to the natural migratory 

 power of the beetle. Immediately after crossing the Missouri River and 

 entering upon the prairie country the influence of winds upon the dissemi- 

 nation of L. decemlineata was clearly shown in the rapid advance to the 

 northeast into Wisconsin. This advance was, as shown on plate 8, in 

 general coincident with the direction of the wind. Topographic barriers 

 and natural highways were absent, the wind serving almost, if not entirely, 

 as the controlling factor in that region. In the Mississippi Valley, also, we 

 find that the rapid northward advance was in part due to the existence of a 

 natural highway and in part to the influence of the wind. 



It is not always that winds have been favorable to the migration of this 

 beetle. Whereas in the above cases aid of a substantial kind was given, in 

 others it has acted as a strong retarding agent, the best examples of this 

 being found in the history of the southward advance. Tiie migration down 

 the Mississippi Valley was, as we have seen, slow, owing almost entirely to 

 the fact that there is a strong indraft of air northward during the summer 

 months. How effectually this wind served to retard the migration may be 

 indicated by the fact that it required about 32 years (1867-1900) to cover 

 the territory from Cairo, Illinois, to New Orleans. The slowness of this 

 advance appears still more striking if we compare it with the migration 

 from the southern end of Lake Michigan to the Atlantic coast, a distance 

 about equal, which required seven years. In other words, fourteen genera- 

 tions were required for the beetle to advance from Lake Michigan to the 

 Atlantic coast, and more than sixty to cover the distance between Cairo and 

 the Gulf of Mexico. Likewise, the dissemination over the Atlantic Coastal 

 Plain and the Piedmont belt has been retarded to a considerable extent by 

 adverse winds. 



Cyclonic disturbances often transport birds and strong-flying insects long 

 distances from their native habitats, and these, coming to rest upon a distant 

 land, might under favorable conditions be able to start a colony. But while 

 numerous cases occur every year of the flight for hundreds of miles of strong- 

 winged Lepidoptera and Orthoptera, when aided and controlled by the 

 wind, we do not yet know of a single instance in which this flight has 

 resulted in a permanent dispersion. For example, Erebus odor a, a strong- 

 winged noctuid, which breeds in southern Mexico and southward, is found 

 in numbers every year in the United States, and to some extent in Canada. 



4— T 



