29 



north and west towards the primal habitat. This has been proved by repeated obser- 

 vation. One reason for this is found to be the prevalence of favourable winds at that par- 

 ticular season in the regions where these locusts are produced ; for locusts, and indeed, 

 all migratory insects, are dependent to some extent upon the winds for assistance and 

 direction in their migrations. This is true for locusts all over the world ; they are brought 

 by the wind and taken away by the wind. A striking instance of this fact is given in 

 the account of the great Egyptian plague of locusts in the Book of Exodus. 



So with our American migratory locust. The general direction of the winds on the 

 eastern slopes of the Rocky Mountains and on the plains is, during July and August, 

 west or northwest. These are the months during which the locusts come down from their 

 mountain home to invade the cultivated plains of the border States. And when the 

 generation of which these are the parents attain the winged state, in the following June, 

 it has been found that the prevailing winds are from the south and southeast, and thus 

 are favourable to the flight of the locusts in a northerly or westerly direction. 



As regards their powers of flight, it has been proved by experiment that the locust, 

 when it has a favourable wind (and it rarely flies at any other time), does not fly faster 

 than the wind, but merely uses its wings to sustain itself in the air, and allows the breeze 

 to waft it along. An observer proved this by ascending to the top of the State Univer- 

 sity of Nebraska when a swarm of locusts was passing, and letting loose among the flying 

 grasshoppers small bunches of cotton. He found that the cotton sailed along quite as fast 

 as the grasshoppers did. 



Their numbers are inconceivably great. A British officer who saw a swarm in Syria 

 estimated their number at 180,000,000,000,000. The clouds of them seen in the west 

 have often exceeded 50 miles in length by 20 in breadth, with a depth of from a quarter 

 of a mile to a mile ; 1,500,000 bushels of their dead bodies were estimated to be lying on 

 the shores of Salt Lake, in Utah, after a visitation of their hordes. And their eggs are 

 found in the ground in numbers of from 100 to 15,000 to the square foot, in localities 

 favourable to their deposition. Such are some of the reliable statistics gathered regard- 

 ing the Bocky Mountain locust. 



A curious and fortunate fact with regard to the locust is that it does not become ac- 

 climated in the regions to which it migrates. The hordes from the north, fresh from the 

 invigorating air of the mountains, are much stronger and more vigorous than their pro- 

 geny born the succeeding year in the plains of Missouri and the other Western States, 

 Professor Aughey, of the State University of Nebraska, tested their muscular strength by 

 attaching their hind legs to a delicate spring balance and observing the degree of strength 

 they exerted. He invariably found that the locusts from the mountains were stronger 

 than those born in the plains. He also found that the mountain insects could live with- 

 out food for several days longer than the others. Their eggs are also injured by the 

 moister climate, so that it is estimated that fully one-half become addled and never hatch. 

 These circumstances tend to so reduce their numbers in the new habitat that in a few 

 years the species dies out. 



This locust is a near relation of our common Canadian 

 locust (Caloptenus femur-rubrum), fig. 11. The latter has 

 often been injurious to the crops, particularly of grass 

 and hay, but has little tendency to migrate. It has a vast 

 range, from Labrador to the Pacific coast, including the 

 Western States and Mississippi Valley as far south as 35°. 

 Leaving the locusts, we will pass to the more pleasing duty of noticing some migra- 

 tory insects w^hich are comparatively harmless, and are far more beautiful than any of the 

 Orthoptera. 



Many of the butterflies are inclined to migrations, particularly the whites and yel- 

 lows ( Pier is, Colias and Callidryas ). These genera, with a few exceptions, are not very plen- 

 tiful in temperate regions, but have their home in warm climates. So from equatorial and 

 South America, and from the southern parts of Europe have come reports of vast migra- 

 tions of these butterflies. Bates, in his "Naturalist on the Biver Amazon," gives an in- 

 teresting account of the uninterrupted procession of butterflies belonging to the genus 

 Callidryas which he saw passing from morning to night in a southerly direction across the 



