370 THE CANADIAN NATURALIST. [Vol. viii. 



immense treeless plains from the northwest, often at the rate of 

 fifty or sixty miles an hour, the darkening locust clouds are soon 

 carried into the moist and fertile country to the southeast, where 

 with sharpened appetites they fall upon the crops, a plague and 

 a blight. Many of the more feeble or of the more recently fledged 

 perish, no doubt, on the way; but the main army succeeds, with 

 favourable wind, in bridging over the parched country which 

 affords no nourishment. The hotter and dryer the season, and 

 the greater the extent of the drought, the earlier will they be 

 prompted to migrate, and the farther will they push on to the 

 east and south."* 



We have, second, the return migration toward the northwest 

 from the country south and east of the lines already indicated, of 

 the progeny of invading swarms, as soon as wings are acquired 

 the next summer. Time will not permit me to present the ex- 

 planation of this return migration. In the work just quoted I 

 have discussed its causes, the reasons why the species cannot per- 

 manently thrive in the Mississippi valley, and the conditions 

 which prevent its establishment there. 



We have, third, the eastern limit of the insects' spread along 

 a line broadly indicated by the 94th meridian, and the consequent 

 security from serious injury east of that line. 



These three features of our disastrous swarms — the return 

 migration from the southeast country (which implies only tem- 

 porary injury therein), and the eastern limit, — may be stated 

 as laws governing the insect east of the Rocky Mountains. They 

 have constantly been urged by me, and the present year's expe- 

 rience has confirmed and verified them. I think I may safely 

 present a fourth, namely, that the eggs are never laid thickly two 

 successive years in the same regions. 



In mapping out the country in Kansas and Missouri in which 

 eggs had been laid most thickly in 1876, I was struck with the 

 fact that the very counties in which the young insects had been 

 most numerous and disastrous in 1875 were passed by or avoided, 

 and had no eggs of any consequence laid in them in 1876. The 

 fact was all the more obvious because the insects did much dam- 

 age to fall wheat, and laid eggs all around those counties, to the 

 north and south and west. From the exhaustive report on the 



* Locust or Grasshopper Plague, page 57. Band, McNally & Co., 

 Chicago. 



