55 



distinctly whenever larvse of any kind are crawling along, characters 

 drawn from the legs are evidently the most generally available. 



The number of distinct species of Mites to be found in the 

 United States is rather large, but still very greatly inferior to the 

 number of distinct species of Insects found within the same limits. 

 Unlike the true Insects, they are all of them quite small in size, the 

 largest Mite known to me being much less than one-tenth inch in 

 length. They swarm everywhere; but their Natural History is so far 

 almost entirely a sealed book to the scientific world, because no one 

 hitherto has devoted his exclusive attention to them. I am myself 

 acquainted with no less than twenty distinct species, that form curious 

 "galls," or unnatural growths, upon the leaves of various trees. One 

 of these causes the singular pod-like growths, about one-eighth inch 

 long, . upon the upper surface of the leaf of the wild plum-tree 

 {Primus antericana)* which often swarm so prodigiously, that I 

 calculate that the number of young Mites, in one small clump of 

 Plum-trees, frequently exceeds the number of human beings now liv% 

 ing and breathing upon the face of this earth. This will perhaps 

 be considered a wild exaggeration; but see what the figures them- 

 selves will say. I have often counted as many as sixty of these galls 

 on a single leaf, and each gall contains towards the end of July sev- 

 eral scores of microscopically minute young Mites. Such a leaf will 

 therefore contain about 3,000 young Mites, and putting the human 

 population of the whole globe even at the enormous number of 

 900,000,000, it will only take 300,000 such leaves to verify my esti- 

 mate. Now, Dr. Fitch has calculated {New York Reports I. p. 137) 

 that there are about 17,000 leaves on a yoimg cherry-tree only ten 



*For the benefit of the scientific reader, I copy from my Journal the de- 

 scription of this one gall. The general reader will be thankful that I omit 

 the descriptions of the other nineteen galls: 



Gall Pruni crLi'MENA, new species. On Prunus americana. A fleshy, 

 smooth, elongate, blunt-tipped, fusiform, opaque, hollow gall, constricted at 

 its base, and with a few erect hairs, 0.10 — 0.16 inch long, and about four or 

 five times as long as its extreme breadth. Walls of the gall thin. Color 

 outside a very pale green often tinged with rosy; inside, rough and of a 

 rosy color. Always grows upon the upper surface of the leaf, whole trees 

 frequently swarming with it, the number of galls on a single leaf varying 

 from one to sixty. Ten galls opened July 27th all contained Acarus larvse, 

 scores of them in each gall. These larva are exceedingly minute, of a hya- 

 line-whitish color, of the usual elongate-oval form, thrice as long as wide, 

 six-legged, with their legs arranged as usual. They are very sluggish. Some 

 of a yellow color were crawling- on the leaves outside the galls. A similar 

 but distinct gall (Ccrasi crumena, Walsh MS.) is almost equally abundant 

 on the leaf of the Wild Black Cherry (Cerasus serotina.) 



