500 BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



most common species iti this section (Southern Illinois), and could, 

 throughout the summer and fall and even during the spring, be found 

 at any time in the fields and along the highways ; but, strange to say, 

 this season that form has entirely disappeared, and has been replaced 

 by a rather more slender form, with the last segment distinctly notched 

 and the wings lengthened, resembling, and apparently identical with, 

 Professor Eiley's C. atlanis. How are we to account for this? It will 

 not do to call it a hybrid between the spretns and the femur -ruhrum, as 

 the former has never been known to visit this region, at least in num- 

 bers sufficient to attract attention, the great army last season having 

 penetrated but a short distance into the western side of Missouri. Nor 

 will it do to say my examinations have not been sufficiently thorough, 

 for I have kept watch of them during the entire summer, gathering 

 hundreds, and, although finding some variation, have failed so far to 

 find a single femur-riibrum. 



I am also informed, by a letter just received from Professor Burril, of 

 the Industrial University at Champaign, in this State, that since he 

 noticed an article I recently published on this subject, he has paid some 

 attention to the matter, and finds the same thing true there. I also ob- 

 serve a note in the last number of the American Naturalist, from Dr. 

 Packard, mentioning the occurrence of spretus (probably atlanis) in Mas- 

 sachusetts. Also the very fact that Professor Eiley last year mentions the 

 intermediate form, which he names as a new species, and which had never 

 before attracted attention, coincides with the other facts I have men- 

 tioned. Here, then, beyond dispute, a remarkable change is taking 

 place, which gives rise to a number of important questions. And first 

 of these is. What is the cause of this 1 I think it is owing chiefly, if 

 not entirely, to climatic influences, and forms an index to the great 

 changes in specific characters which may be effected by a change of 

 climate. If I am correct in this, it follows that when the climate re- 

 verts to its normal condition the species will do the same ; and, on the 

 contrary, if one should be permanent the other will also, in all proba- 

 bility, be the same. 



I may also notice, as bearing upon this point, the fact (for since the 

 publication of my recent article I have ascertained it is a fact) that 

 Caloptenus differentialis Thos. has been seen in the central part of this 

 State flying in bodies at considerable height, and apparently mi^j'rating. 

 Acridium emarginatum Uhl., a Western species, never before known 

 to occur east of the Mississippi, has been discovered this season as far 

 east as Blooraington in this State. These facts are certainly important 

 and instructive, and deserve careful consideration. 



The chief practical questions connected with this subject, and which 

 more directly concern our agriculturists, are these: — (1) Will invasions 

 of the C. spretus grow more and more frequent ? (2) Will it continue to 

 extend its limits farther and farther eastward! (3) Will the changes 

 now taking place result in producing migratory hordes in our midst? 



