1877.] The Destructive Locust of the West. 23 
cust of the West. The dates which are starred are years when 
the progeny of the locusts of the preceding year abounded, and 
when in most cases there were no fresh incursions from the west- 
ward. The species referred to under the head of California, 
Washington, and Oregon may be some other than Caloptenus 
spretus. 
weda iine E Wad Ria eee Indian | Washington 
anitoba. and and K ade BET HES and 
Western | Dakota. | Idaho. | 7 | Tado: | Western | and `| nia. | Oregon. 
1818 1818 1827 or 728 
1819 1819 1834 or ’35 
1820 1820 or ’21 1838 
r 1845 1845 
1846? 
1852 1852 1849 1852 
1855 1855? | 18557] 1855 | 18552 1855 1855 1855 1855 
1856* 1856* 1856* | 1856 
1857 1857 
1864 1864 1864 1864 
1865* 
1866 1866 
. 1867 1867 1867 | 1867 1867 1867 
1868* 1868*| 1868 1868* 
1869 1869* 
1872 
1873 1873 1873 18732) 1873 1873 
1874 yen 1874 | 1874 1874 1874 1874 |South. 
f 1875 | 1875 | 1875 A 1875*| 1875* | 1875 Cal. 
1876 1876 1876 1876 1876 1876 
This table and the data on which it is based are necessarily 
very imperfect, owing to the vast extent of the territory over 
which the locust swarmed, and the fact that the greater portion 
is uninhabited, while the inhabited portions have been settled 
only within comparatively few years. 
The Theory of the Migrations. — (1.) The immediate cause of 
the migrations of the locust from its original breeding places is 
the unusual abundance of the species during certain years. It 
has been found in some cases that the exceptional years when the 
locust migrates are periods of unusual heat and dryness, condi- 
tions unusually favorable to the excessive increase of insect life. 
8 may be seen in the accounts of the eastern locust, the grass 
army worm, the grain aphis, the chinch bug, and other less de- 
structive insects, when the early part of the season, the spring — 
and early weeks of summer, are warm and dry, without sudden 
changes of temperature, insects abound and enormously exceed 
their ordinary numbers. When two such seasons occur, one after — 
the other, the conditions become still more favorable for = undue oe 
