204 REPORT UNITED STATES ENTOMOLOGICAL COMMISSION. 



the locust-life, which ia itself directly or indirectly influenced by the weather at every 

 stage, from the deposition of the egg through the life of the young and the mature 

 insect, up to the period of its migration and final deposition of new eggs in a new 

 locality. 



From the data given in the preceding part of this chapter it has been made probable 

 that locust visitations are coincident with years of little rain, but it still remains an 

 open question whether the weather has directly affected- the insect or only indirectly 

 through its influence upon the crops and the parasites. It is difficult to separate these 

 two modes of action without going into minute details as to the relations between the 

 daily life of the insect and the weather actually prevailing; all generalities as to 

 monthly or annual means of temperature, rain-fall, winds, &c., or as to the sun-spot 

 period, as affecting the growth and migration of the locust, are very likely to contain 

 errors due to our own want of a correct guiding theory, based on a knowledge of the 

 peculiarities of the locust. 



It would be perfectly practicable to prepare a series of daily maps showing tho 

 weather, winds, &c., of each afternoon over the locust areas, and on these maps also 

 graphically present daily systematic reports of locust-life, migrations, &c. This is the 

 most thorough method of searching for the general laws that we are now desirous to 

 find. A series of such maps and reports would, in one season, show most positively 

 what atmospheric features directly, and what indirectly, aflect the locust. In the ab- 

 sence of such minute study, I offer the following result : 



I note the following three epochs of locust migration, i. e. : 



1. A slow change of locality during the hottest portion of the day, evidently dictated 

 entirely by the scarcity and search for food; this occurs while unfledged (before tho 

 fourth molt — Riley's Report, 1876, p. 100), and is especially active during May and a 

 part of April. 



2. Later in their life occurs the grand migratory movement of large bodies in a uni- 

 form direction, and to considerable distances. This is repeated daily from 9 a. m. to 

 sunset, if the weather is favorable, and is followed by deposition of eggs in the new 

 country and the death of the female. This migration takes place within two weeks 

 after the fourth molt, and continues through June and July. 



.3. The broods hatched out in foreign or low lands apparently attempt, when their 

 migration begins, to return to the original breeding-grounds near the Rocky Mountains. 



In general, the most important climatic element affecting migration is believed to 

 be the wind. It will, however, not do to confine ourselves to the "prevailing wind," 

 by which is technically meant that one which has occurred most frequently during 

 the month, nor to the mean movement, nor to the resultant movement of the wind, 

 but we must study the relative frequency of the winds from each point of the compass 

 and their individual characteristics as to relative humidity, temperature, velocity, 

 rain-fall, &c. 



I have, therefore, compiled the following tables of wind-frequency for May to Sep- 

 tember and for 1874, '75, and '76, from the summaries published in the annual reports 

 of the Chief Signal Officer for 1873 to 1877, inclusive. 



In future investigations the tables may be enlarged by addition of the numerous 

 stations now maintained in Texas, for which the wind-frequency is not given in the 

 annual reports referred to, for the years 1874, '75, '76. 



