70 



where only the Oyster-shell Bark-louse can thrive — as for example 

 in Northern Illinois and Wisconsin — it is death upon apple-trees, for 

 6 or 8 years after it is introduced, but afterwards sobers down, and 

 though still a grievous pest, becomes comparatively speaking, in- 

 nocuous. 



Before concluding this long chapter, I ought to caution the reader 

 against a very prevalent, but a very delusive idea. People are per- 

 petually reasoning upon the assumption, that any fluid substance, that 

 they may apply to the limbs of a tree, is taken up by the sap and 

 carried to the remotest twig; as if plants, like the higlier animals, 

 had a complete circulatory system of veins and arteries ; whereas every 

 botanist knows that it is no such thing. "Whatever you apply to your 

 tree to kill the Bark-lice, whether soapy solution or oily fluid, can 

 only kill those insects that it actually touches, and will not be ab- 

 sorbed by the sap and carried unchanged to other parts, so as to kill 

 the Bark-lice upon those parts. If it were otherwise, the apples on 

 a tree that had been soaped would taste of soap, and those on a tree 

 that had been treated with kerosene, would taste of kerosene. But 

 that this is not so, every one may satisfy himself by an easy experi- 

 ment, if he does not, as I do, know the fact already. Possibly, to a 

 very limited extent, such substances as those referred to above may 

 be absorbed by the cellular system of the tree; but even in that case 

 they will undergo chemical changes which will totally unfit them for 

 destroying insect life. To believe that pure kerosene, or pure soap, 

 applied to one end of a tree, will pass in the very same chemical form 

 to the other end of it, is as absurd as to believe that liquid manure 

 can be taken up by the roots of a tomato-plant, and pass in that form 

 and without any chemical change into the ripe tomatoes. 



CHAPTER IX. — Harris's Bark-louse. {Asptdiotus Harrisit, Walsh.) 



I have discussed the Natural History of the Oyster-shell Bark- 

 louse at such exorbitant length, that it will not be either necessary or 

 advisable to dilate upon that of this species, further than to point out 

 the very remarkable characters in which it differs from the other. 



1st. The diflerence in the shape and color of the scales, and in 

 the color of the eggs, has been already explained. (See above, page 

 46.) The eggs hatch out at almost exactly the same date, (June 5th, 

 1867,) but, instead of the young larvae being yellowish white, and 

 soon afterwards becoming covered with a white powdery bloom, so 

 as to form conspicuous although very minute wliite objects on the 

 bark, they are blood-red at first and afterwards blood-brown, without 

 any powdery bloom ; and consequently, from their extreme minuteness 



