200 Tlie RocJcy Mountain Locust. 



scarcely noticed at first, and made little impression on the 

 luxuriant vegetation that a wet and favorable season pro- 

 duced. By September, when a spell of dry weather cured 

 the grass and the locusts had acquired full growth, we can 

 imagine that they swarmed in much of the prairie country 

 of Central Illinois. Whenever they abound to an unusual 

 degree the migrating instinct is developed, just as it is 

 under like circumstances in many other insects, as butter- 

 flies and beetles, that are normally non-migratory. 

 The reasons we can only surmise ; but aside from 

 those of hunger, etc., previously suggested, the an- 

 noyance and inconvenience to which the females, while 

 attempting to oviposit, have to submit from their com- 

 panions, under conditions of excessive increase, may 

 have something to do with it. But mere increase in 

 numbers would not give to specks like femur -rubrum 

 and differentialis, which are ordinarily heavy-bodied 

 and short winged, the power of extended flight ; and 

 there is little doubt, in my mind, that the same excep- 

 tionally hot, dry seasons which permit this undue multi- 

 plication also modify the individuals, and cause a decrease 

 in bulk and increase in wing-power. The facts support 

 this view, for the flying specimens of differentialis sent to 

 Prof. Thomas had, as he writes me, "the body lighter 

 and the wings longer, and some of that peculiar fierce ap- 

 pearance belonging to migrating specimens ; " and I have 

 specimens from Kansas and Minnesota which differ so 

 much in these respects from the more normal specimens as 

 found at St. Louis in ordinary seasons, that they can scarcely 

 be recognized as the same species. The casual observer 

 knows how thoroughly plants are modified in size and 

 habit by season and condition : the same holds true of in- 

 sects, and more particularly in certain groups. 



Given that over the vast prairie region of Central Illi- 



