45 



passing from point to point in droves, is it not very likely the cer- 

 tainty in the present case explains the uncertainty in the former, and 

 that C. differentially was the culprit in the first case instead of C. femur- 

 rubruin, as Mr. Walsh supposed, or Acridiwm Americum (in Southern 

 Illinois), as I then supposed from the number, I observed ? 



There is also a coincidence here I would call attention to, as it is 

 another item in the data which will ultimately explain to us things 

 which are now mysterious. The unusual action of our Acridii this 

 year is during the season following the great invasion of 1874, so that 

 of 1867 was the season following the great invasion of 1866. 



There is no probability that this species (C. differential-is) will ever 

 become habitually migratory as the C. spretus, yet what has occurred 

 once may occur again, and like causes may reasonably be expected to 

 produce like results. I am inclined to think the peculiar seasons we 

 have had for two years, and especially the character of the spring and 

 summer, have had much to do with the sudden development of this 

 species, which is a great feeder and fond of rank vegetation which the 

 summer rains afforded. But why these insects should migrate when 

 food is so abundant is somewhat strange, unless we suppose the recent 

 dry weather (as we have had here) has rendered their food dry and 

 unpalatable, or that something in the condition of the atmosphere so 

 affects them as to urge them on. 



The remarkable effect of the past and present seasons (if the cause 

 lies here), upon our Acridian fauna does not stop with the C. different- 

 ialis, but has extended to our most common species, the Caloptenus 

 femur-rubrum. This species, which can usually be found anywhere in 

 the fields or along the roadside, during the summer and fall, appears 

 to be entirely replaced by a new form, which I take to be the one de- 

 scribed by Professor Riley as Caloptenus atlanis, which is an intermedi- 

 ate form between C. femur-rubrum and C. spretus, so near in fact to the 

 latter, that it is almost impossible to distinguish the one from the 

 other. I have searched in vain for femur-rubrum ; it seems entirely to 

 have disappeared and the new variety to have taken its place. Is the 

 one the progenitor of the dther ; the former of the latter? I am no 

 believer in the Darwinian theory, but here is presented a problem 

 difficult to solve, unless we admit the correctness of that theory, or, 

 that all three supposed species, are but varieties of one which 1 am 

 inclined to believe is the case. Otherwise how are we to account for 

 the appearance of this new form this season? The spretus has not 

 visited our section, the femur-rubrum is absent, and here I have before 

 me a large number of specimens gathered here, some of them to-day 

 (September 28, 1875) with the long wings and the notched male ab- 

 domen, corresponding exactly with Professor Riley's description of 

 Caloptenus atlanis. Is the common femur-rubrum being transformed 

 into spretus, this being the intermediate step? If so, over half the 

 distance has already been traversed. What has caused this ? Is it 

 some great climatic change that is taking place* or some hidden and 

 hitherto unknown cause? I confess my inability at present to solve 

 this new problem. Further observations and more data being neces- 

 sary. Yet I am still of the opinion that without an entire change of 

 climate the Caloptenus spretus will never become a permanent resident 

 of Illinois or the immediate valley of the Mississippi. 



