DIRECTION; TIME OF APPEARANCE OF INVADING SWARMS. 217 



must be considered the niiuimum rate where there is no impediment. 

 When tacking against the wind, they may move not more than one 

 mile,^° while the maximum rate, in a strong wind, may reach as high as 

 50 miles or more per hour. 



DIRECTION OF INVADING SWARMS. 



While there may be, during an invasion, local flights in all possible 

 directions (except, perhaps, due west), the general movement east of 

 the mountains is conspicuously toward the south and southeast. The 

 more local and irregular flights are generally made for food, but the 

 more extended, southward movements are in obedience to other laws, 

 discussed in the preceding chapter, and also on p. 250. West of the 

 main Eocky Mountain range the rule of flight appears to be from the 

 higher plains and plateaus, where the insect normally breeds, to the 

 lower and more fertile valleys ; and the greater irregularity of the pre- 

 vailing winds and more broken nature of the country preclude the same 

 regularity in directions of flight that, on the whole, prevails east of 

 the range. 



TIME OF APPEARANCE OF INVADING SWARMS. 



"In endeavoring to deduce general conclusions respecting the time of 

 year that the 1874 swarms reached different parts of the country, great 

 difficulty was experienced in sifting those accounts which referred to 

 the progeny of the 1873 invasion, and those which hatched within the 

 insect's native range, and came from the extreme Xorthwest. The same 

 \\as true of the fresh 187G swarms, and those which hatched in Minne- 

 sota." 



As a rule, the insects which hatch in the temporary region acquire 

 wings, and leave before the fresh swarms from the mountain region 

 appear. In the more northern regions, as in Minnesota and Manitoba 

 westward, the insects hatched on the ground acquire wings the latter 

 part of June and in July. The period is earlier as we go south, until in 

 Southern Texas they are able to fly in April. The time of appearance 

 of invading swarms from the permanent region is in inverse ratio, i. e., 

 earlier to the north and later to the south. Thus, while on the confines 

 of the permanent region it is almost impossible to distinguish between 

 the insects which hatch there and the fresh swarms from the Northwest, 

 the difference becomes more and more marked toward the south and 

 east. 



^'In 1874, swarms appeared during June in Southern Dakota j during 

 July in Colorado, Nebraska, and Minnesota -, during the latter part of 

 this month in Iowa and Western Kansas. During August they came 

 into Southeast Kansas and Missouri; and by the middle of October they 

 reached Dallas, in Texas. In 1876 they came later." 



30 Mr. D r. Weymouth, of Lyon, Marshall County, Minn., records their going West iu 1865, in "the 

 teeth of a strong wind, making scarcely a mile an hour." See also p. 160, ante. 



