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that I went scvoval times to tliat tree to contemplate the spectacle which 

 these insects presented. And all tliroug-h the followinjj winter, no poison 

 passinj? within sight of that tree could fail of noticing the blackness of 

 its trunk and linihs, it being the more remarkable as none of the other 

 willow trees around it had any tinge of this color. The thought thereupon 

 became impressed upon my mind, tliat it was the plant-lice with which this 

 tree had been so overrun the preceding summer, which had in some way 

 imparted this blackness to its bark. Two or three winters afterwards, I 

 noticed the same blackened ajipearance to a pine tree, which tree t knew 

 liad been thronged with Aphides the sunmier before. I need not specify 

 the several other instances of this phenomena which I have noticed. Seve- 

 ral years since, when I was investigating the Aphis of the apple tree, 1 

 discovered that, in addition to the bark of trees, the leaves also acquired 

 this sooty appearance, from these insects; and then, upon giving this sub- 

 ject a particular examination, I became assured that this black substance 

 was merely the honey dew in a decomposed state. 



Some writers have remarked that dry weather causes the several kinds 

 of plant-lice to increase and become pests to the diflbrent species of vege- 

 taticm which they respectively inhabit; and my own observations incline 

 me to regard this remark as being correct. During the dry period in June 

 which fre<}uently succeeds the spring rains, I have in particular years 

 noticed these insects as occurring in unusual numbers, wliereupon I have 

 apprehended that, having acquired such a start so early in the season, they 

 would prove to be the most pernicious insects of the year; but rainy weather 

 coming on after this, they have seemed thereupon to decline and have ceased 

 to attract further attention. Hence I think it true as a general rule, that 

 dry weather favors and wet weather retards tlipir increase. And at first 

 thought, this view is further strengthened by the fact that this Aphis upon 

 the hops was so excessively numerous the past summer, when we experi- 

 enced a drouth of such protracted length and severity. But, on the other 

 hand, these insects were simihuly numerous the year before, when the 

 summer was unusually wet. We arc thus assured there is some influence 

 more potent than the hygrometric state of the atmosphere, which has 

 brought them forth in such hosts upon the hops. 



Perhaps in no other group or family of insects are the different species 

 so very closely akin to each other as in this of the Aphides. So nearly 

 identical are most of them, both in their appearance and habits, that we 

 know them to be distinct species only from the fact that they inhabit dif- 

 ferent plants, each one being unable to sustain itself upon any other than 

 the plant to which it belongs. Being thus intimately related, we should 

 confidently expect that the same atmospherical or other influence wJiich 

 causes one species to suddenly multiply and become extremely numerous, 

 would operate upon and similarly affect the other species also. But this 

 is by no means the case. As every one will remember, in the summer of 

 1861, all our fields of grain suddenly became so thronged with the Grain 

 Aphis as to throw the whole country into alarm. Why did not the same 

 cause which brought that insect upon us in such a remarkable manner 

 operate also to bring this insect upon the hops at that time instead of 

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