NO EGGS LAID IN THOROUGHLY DEVASTATED REGIONS. 249 



bensible instincts which guide most animals in the preservation and 

 perpetuation of their species. Why, one single leaf like this from the 

 book of Nature throws a flood of light on the habit which the locusts 

 display of roosting above ground, and of many of their other doings 5 

 and that book is full of just such lessons to those who carefully and 

 intelligently turn and scan its pages. 



What is true of the Locust mite and its influence on the movements 

 and doings of the locust is true, though in a less and varying degree, 

 of the other insect enemies mentioned in Chapter XI, for wherever the 

 locust unduly abounds, there its natural enemies also riot in plenty and 

 rapidly multiply. 



As the rule we have thus stated is a most encouraging one for the 

 farmers of the Border States, if it can be established, we have endeav- 

 ored to get the general experience by requesting replies to the question: 

 "Have you ever known the eggs of this Eocky Mountain Locust to be 

 thickly laid for two consecutive years in the same ground or in the 

 same locality ? " • 



The replies to this question, which we have brought together (App. 

 23), bear out the general statement, for the large majority of the answers 

 are in the negative, and some of the correspondents have lived in the 

 W^est for 20 and 25 years, ^o attempt was made to get numerous re- 

 plies from Colorado and Minnesota, or from the Northwest, because it 

 is well known that the eggs are often laid there for consecutive years. 

 It is equally true, however, that in those sections where the insect may 

 prevail and hold its own for two or more years in succession, it is sel- 

 dom so numerous as to keep the earth bare of vegetation in midsummer, 

 and whenever it is so numerous even in Minnesota, the rule seems to 

 hold true, according to Mr. Whitman's experience, as already given. 



The rule we wish to lay down may, perhaps, be more safely stated 

 thus : That whenever the young insects abound to such an extent as to Jceep 

 the earth hare of vegetation in summer {iinplying thiclc and extensive egg-lay- 

 ing the previous year)^ there will be no eggs laid in such devastated regions 

 the same year. This means that a year of utter locust devastation like 

 that of 1875, in many parts of the West, is pretty sure to be followed by 

 one of perfect immunity from the pests. 



CAUSES OF MIGRATION. 



Many writers on the migratory locusts of the "Old World" and on our 

 own species have, naturally enough, discussed the causes of migration. 

 We are of the opinion that most authors have erred in endeavoring to 

 ascribe these migratory movements to any single cause, and that sev- 

 eral influences play with varying force, and either singly or combinedl3'. 

 We must also recognize the fact that the influences bearing on migra- 

 tion fall into two distinct categories, viz, remote or general, and imme- 

 diate or special. The remote influences are climatic and meteorological, 



