244 REPORT UNITED STATES ENTOMOLOGICAL COMMISSION. 



spretiis could establish itself under similar climatic Inflaences, it would 

 also be digoneutic, and the premature hatching of its eggs in autumn, 

 wtien laid in southerly regions, is nothing but a step toward digoneu- 

 tism. But the record shows that it cannot so establish itself, and that 

 it can breed permannetly under those conditions only which induce 

 siiigle-broodedness. 



THE SPECIES CANNOT PER^VIANENTLY DWELL IN THE TEMPORARY 



REGION. 



Intimately connected with the question we have just considered, is 

 the consideration of the present statement. That the disastrous inva- 

 sions into the Mississippi Yalley, like those of 1866, 1874, and 1876, 

 have their source in what we designate as the Permanent region, is 

 abundantly proven from what has preceded in Chapters II and Y. It is 

 matter of common observation that the insects that participate in such 

 invasions are longer-winged and paler than those which develop in the 

 Temporary region, and this fact accords with the records, in placing 

 their origin in the high dry plains regions of the Northwest j for natu- 

 ralists find it to be a very general rule that paleness of color and increase 

 of size and of wing-power are characteristic of animals inhabiting the 

 said region. 



The comparatively sudden change from the attenuated and dry at- 

 mosphere of the elevated plains and plateaus which constitute the Per- 

 manent region to the more humid and low prairie region of the Missis- 

 sippi Valley proper, is injurious to the species, though its conse- 

 quences are not manifest with the invading insects, except, perhaps, in 

 limiting their eastward progress. The first generation, however, 

 hatched in the low, alluvial country, is more or less unhealthy, and the 

 insects do n,ot breed here, but quit the country and get back, as far as 

 they are able, to more congenial breeding-grounds. If the weather be 

 particularly wet and cold they perish in immense numbers, and there is 

 every reason to believe that even the bulk of those which attain maturity 

 are intestate and perish without procreating, because the large major- 

 ity of those which drop on the return to the Northwest contain no eggs. 

 Ill the Subpermanent region, or as we go west and northwest, the spe- 

 cies propagates, and becomes localized more and more until we reach the 

 country where it is always found. Nothing is more certain than that the 

 species is not autochthonous in Texas, West Arkansas, Indian Terri- 

 tory, West Missouri, Kansas, Western Iowa, Nebraska, or even Minne- 

 sota ; and whenever it overruns any of those States, it sooner or later 

 abandons them. 



" We may perhaps find, in addition to the comparatively sudden 

 change from an attenuated and dry to a more dense and humid atmos- 

 phere, another tangible barrier to the insect's permanent multiplication in 

 the more fertile country to the southeast, in the lengthened summer sea- 

 son. As with annual plants, so with insects (like this locust) which pro- 



