STATE AGRICTTLUURAL SOCIETY. 113 
these insects presented. And all through the following winter, no person 
passing within sight of that tree could fail of noticing the blackness of 
its trunk and limbs, 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, that 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 appearance to a pine tree, which tree I knew 
had been thronged with Aphides the summer 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 different species of vege¬ 
tation which they respectively inhabit; and my own observations incline 
me to regard this remark as being correct. During the dry period in June 
which frequently succeeds the spring rains, I have in particular years 
noticed these insects as occurring in unusual numbers, whereupon 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 their 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 similarly numerous the year before, when the 
summer was unusually wet. We are 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 which 
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 
two years later ? Or, if this insect was not then in our country, when it 
did appear in such vast numbers two years ago, why was not the same influ- 
[Aq. Trans. j 8 
