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lice. And I have further observed, that it is the lower horizontal 

 limbs and branches, or such as birds, with the exception of Wood- 

 peckers and Nut-hatches, would most naturally perch on, that are 

 first attacked. The process of transmission, however, is by no means 

 so sure and speedy as in the case of winged insects — for example, the 

 Plant-lice {Aphis family.) For every one must have often noticed 

 trees standing not far from one another, some of which were swarm- 

 ing with Bark-lice while others were not in the least infected. If all 

 the birds in the world were killed off, I believe that these Bark-lice, 

 in a very few years, would cease to exist. They would first of all 

 destroy the trees of which they had already got possession ; and then 

 they would all of them die 6f starvation themselves. As to the pop- 

 ular idea that all Bark-lice crawl along the ground from one tree to 

 another, that is altogether out of the question. They only possess 

 the power of crawling for a few days, and they crawl so exceedingly 

 slow, that I do not believe that in that whole time they could make 

 more than a few yards, even on a perfectly smooth surface. Is it 

 likely, then, that they can ever crawl down the trunk of their own 

 tree, make their way over many yards of groimd which is always 

 moi'e or less rough, and then crawl up the trunk of another tree and 

 pass along on to its branches? 



Mites {Acarus family) are not true Insects, but belong to the 

 same Class (Arachnida) as the Spiders and the true Ticks, as dis- 

 tinguished from the so-called Sheep-tick, which is a wingless true In- 

 sect and — if the hibernicism may be pardoned — a wingless Two- 

 winged Fly (Order Diptera). In common with the rest of the Class 

 to which they appertain. Mites differ essentially from all the true In- 

 sects in having the head and thorax all in one piece, without any free 

 joint or even any suture between them. They differ further, almost 

 ail of them, in having eight logs in the perfect state; whereas all 

 true insects without a single exception have in the perfect state exactly 

 six legs, never more and never less. In very many genera of Mites,how- 

 ever, as in certain genera of Insects, the first pair of legs are not used 

 in walking, but are constantly vibrated up and down as they progress, 

 after the fashion of antennae. Now in Insects, where the head and 

 thorax are always distinct, as the antennse always grow out of the 

 head and the legs out of the thorax, there can be no possible confu- 

 sion between the leg and antenna, no matter what the function of the 

 leg may be ; because, if the organ in question grows out of the thorax, 

 it is to be considered as a leg, even if it discharges the duties of an 

 antenna. But in Mites, where the head and thorax are confounded 

 together, just as they are in a Crab, or a Lobster, or a Crawfish, the 



