448 REPORT UNITED STATES ENTOMOLOGICAL COMMISSION. 



plication of native species, wbicb had, in fact, been quite troublesome 

 in 1874. In 1875, myriads were batched ; but a wet spring and summer 

 produced luxuriant vegetation, upon which these armies made at first 

 but little impression. The waning summer found these locusts ^'in full 

 feather," and the autumn opening with dry weather, wbich cured the 

 grass, the. locusts swarmed in much of the prairie country of Central 

 IlHnois. We found them so abundant, in many places in the vicin- 

 ity of Saint Louis, that several hundred would rise at every step as one 

 walked through a field. Whenever they abound to an unusual degree 

 the migrating instinct is developed in the commoner locusts just as it is 

 under like circumstances in many other insects, as butterflies and beetles, 

 that are normally non-migratory. But mere increase in numbers would 

 not give to species like femur-rubrum and differ entia lis ^ which are ordi- 

 narily heavy-bodied and short- winged, the power of extended flight; 

 and there is little doubt .that the same exceptionally hot, dry seasons 

 which permit this undue multiplication also modify the indivictuals, and 

 cause a decrease in bulk and increase in wing-power. This view is sup- 

 ported by facts, as specimens of differentialis from Kansas and Minne- 

 sota, upon comparison with normal specimens as they are found in our 

 gardens, can hardly be recognized as the same species, so greatly do 

 they differ. The flying specimens of this species that went to make up 

 the Illinois swarms, also had the body lighter and the wings longer, '* and 

 some of that peculiar fierce appearance belonging to migrating speci- 

 mens," to use Mr. Thomas's language. We know that plants are modi- 

 fied in size and habit by changing conditions of season, &c., and the 

 same holds true of insects, though in some groups this susceptibility to 

 modification is more marked than in others. Yet the flight of these 

 species is never so strong nor so long sustained as that of the true mi- 

 gratory species of the Eocky Mountains. 



"In short, whenever the climate and conditions in the Mississippi Val- 

 ley approach those existing in the native home of the Eocky Mountain 

 locust, some of our native species, and especially those nearest akin to 

 it, also approach it in habit. If the climate of Illinois and Missouri 

 were to permanently change in that direction, these species would be- 

 come permanently modified 5 but as there is no immediate danger of 

 such a contingency, the Eocky Mountain locust is the only species, here 

 considered, that can properly lay claim to the migratory habit." 



Another large species that occasionally acquires the migratory habit 

 is worthy of mention here, particularly as it has a wide range and is 

 common every year over the larger part of the United States. This is 

 Acridium ammcanwm, which is figured and described on page 236 (Fig. 6). 

 In 1876, this species was very abundant and in many cases acquired the 

 migratory habit. Dr. S. Miller, of Franklin, Mo., wrote us that swarms 

 passed over parts of Johnson County late in September. The following 

 extracts also refer to this species : 



I send you by Mr. Shaw a small package containing specimens of locusts, destruc- 

 tive about Chattanooga and in all Eastern Tennessee. They strike me as nearly allied 



