642 REPORT UNITED STATES GEOLOGICAL SURVEY. 



the exceptional years when the locust migrates are periods of unusual 

 heat and dryness, conditions unusually favorable to the excessive in- 

 crease of insect life. As may be seen in the accounts of the eastern 

 locust, the grass army-worm, the grain-aphis, the chinch-bug, and other 

 less destructive insects, when the early part of the season, the spring 

 and early weeks of sumnjcr, 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 con- 

 ditions become still more favorable for the undue development of in- 

 sect life. Now it is well known that in the Eastern States the summers 

 of 18G0 and 1874, preceding the appearance of the army- worm and 

 giaiu-aphis, were unusually warm and dry, and favorable not only for 

 the hatching of the eggs laid the year previous, but for the growth and 

 development of the larvae or young. Look now at the conditions for 

 the development of locust life on the hot and dry plains, chiefly of Da- 

 kota, Montana, Wyoming, and Idaho. W^e have no extended meteoro- 

 logical records from these regions at hand, but it is more than probable 

 that the years preceding the migrations of the locusts were exception- 

 ally warm and dry, when the soil was parched with long sustained 

 droughts, as we know that the corresponding 8i)ecies east of the Missis- 

 sippi Itiver abounds during dry summers following dry and warm springs. 



(iiven, then, the exceptional years of drought and heat and the great 

 extent of territory, and we have as the result vast numbers of young 

 hatched out. The year previous having ])erhaps been warm and dry, 

 the locusts would abound, and more eggs than usual would be laid. 

 These would with remarkably few exceptious hatch, and the you tig soon 

 consume the buffalo grass aud other herbage, and move about from one 

 region to another, following often a determinate course in search of 

 food. In this way large broods may migrate a long distance, from per- 

 haps twenty to fifty miles. In about six or seven weeks they acquire 

 wings. Experience shows that the western locust as soon as it is fledged 

 rises up high in the air, sometimes a thousand feet or much higher. 

 They have been .^een to settle at night on the ground, eat during this 

 time, and toward noon of the next day fill the air again with their 

 glistening wings. As more and more become fledged, the vast swarm 

 exhausts the supply of food, and when the hosts are finally marshaled, 

 new swarms joining perhai)S the original one, the whole swarm, possibly 

 hundreds of miles in extent, begins to fly off, borne by the prevailing 

 westerly and northwesterly winds, in a generally easterly aud south- 

 easterly course. 



(2.) The secondary cause of the migration is the desire for food, and 

 ■posHihhi the reproductive instinct. The fact that in their migrations the 

 locusts often seem to select cultivated tracts, rapidly cross the treeless, 

 barren plains, and linger and die on the prairies and western edge of 

 the fertile valleys of the Missouri and Mississippi, indicates that the im- 

 pelling force is due primarily to the want of food, and that the guiding 

 force is the direction of the prevailing winds, for they have no leaders, 

 and we do not believe in the existence of a " migratory instinct" in the 

 locust any more than in the grass army-worm, or the cotton ariiiy-worm, 

 which it is sufficiently evident migrate from field to field, simply in 

 search of more abundant food.* Meanwhile the reproductive system of 



*The simple fact tbat tb« more extensive mij^nilions of the locust both in the New 

 and Old World are periodical, long intervals existing between them, su^jsests'thab the 

 development of a migratory instinct would be impossible. If once partially implanted, 

 the long succession of non-migratory years would etiectually break up the germs of 

 such an instinct. It may be quite diti'erent with birds, which perform their annual 

 migrations for years aud perhaps centuries without fail. 



