26 OYSTER-SHELL BAKK-LOUSE. 



or more ; and we often find them in isolated situations and under 

 circumstances which prechide the idea that they could have been 

 brought hither by human agenc}'. For exanr.ple, the Oyster-shell 

 Bark-louse, being an imported insect, is never found, in a state of 

 nature, upon our native crabs, and when they accidentally get a 

 foothold upon them they do not multiply much, the crab tree be- 

 ing ev^idently uncongenial to their tastes. Yet I have seen them 

 in small numbers upon every tree of a small grove of crab-apples 

 which I have known for twenty-live years, standing upon the 

 prairie, nearly a quarter of a mile from any apple orchard ; and I 

 have found a few scales on another crab-apple tree standing in 

 the edge of the timber, more than half a mile from any cultivated 

 trees. The bird theory, however, is a very inadequate one to ex- 

 plain the general diffusion of the Bark -louse. The most we could 

 expect would be that a few of the insects might possibly be thus 

 transported during the short time when they are not attached to 

 the tree ; and a series of experiments which I made, the past sea- 

 son, with the view of testing this theory, go to disprove the sup- 

 position that they are ever disseminated in this way. I wished 

 to see whether these minute insects would readily crawl on to any 

 obstruction like the toes of a bird. I experimented for hours by 

 putting little slivers of wood about as large as a bird's claw in the 

 way of the crawling Coccids, but in no instance would they crawl 

 on to them. If the stick were put down abruptly before them, at 

 a short distance, say one-tenth of an inch, they would usually stop 

 and turn oif in another direction, showing that they have the 

 sense of sight. If it were put a little further off, so that they 

 would approach it gradually, they would sometimes turn away 

 before reaching it, and at other times they would come up to it, 

 run along parallel to it, and if they could not find a place to get 

 under it, they would turn away. I then beveled off the stick to 

 a sharp edge, so as to present the least obstruction, and at the 

 same time offer an inclined plane up which they could easily crawl, 

 but they would not go on to it. It is pretty evident, therefore, that 

 these creatures have no instinct which leads them to avail them- 

 selves of such means of transportation. 



The third theory is that they are blown from place to place by 

 the wind. That they are carried by a moderate breeze, during 

 their hatching period, to a distance of several rods, has been 

 abundantly proved by myself and by others. If you suspend an 



