39 [ 459 I 



as in the case of young elm trees infested with the cock's-comb gall, and 

 the cotton- wood sometimes has quite a sickly aspect from the profusion 

 of the galls now imder consideration. 



It is one of the admirable i)rovisions of nature that those insects which 

 are stationary the greater part of their lives, such as the scale-insects, 

 and the gall-insects, not only select, with wonderful discrimination, each 

 its own particuliar kind of tree, but in some instances difl'ereiit insects, 

 and sometimes even closely allied species, exhibit a strong predilection for 

 some particular part of a tree, and different from that selected by others. 

 For example, the scale insects of the genus Afytilaspis, to whicb the oyster- 

 sliell bark -louse of the ai)ple tree belongs, are generally found much the 

 most abundantly on the lower branches. This is exemplified by 

 the apple bark-louse, and still more strikingly by the Mytilaspis of the 

 l)ine. In curious contrast with this is the habit of a closely allied 

 species, specimens of which were sent to me by Prof. Uhler, of Balti- 

 more, which were found upon the ornamental linden trees in that city, 

 and which were so exclusively coufined to the ui^per portions of the 

 trees, that he obtained his specimens only from branches which had been 

 brought to the ground by pruning. 



A similar distribution of species occurs, as we have already had occa- 

 sion to observe, in the gall-making Aphides of the cotton-wood, one of 

 which (the vayahundus) is often seen upon the topmost twigs, whilst the 

 species now under consideration {the popnlicaulis) is mostly confined to 

 the lowest branches. This is only one exemiilification of the law of dif- 

 fusion of species, in nature, wdiereby all irdvts of her domain become 

 peopled by living and sentient beings. It is e^^dent also that this lo- 

 calization of particular species may sometimes afford us irai)ortant prac- 

 tical suggestions by gi\'ing to om- remedial measures a more definite 

 aim. When the insect to be combatted locates itself, as in the present 

 instance, upon the foliage of the lowest branches, the obvious sugges- 

 tion is to reduce the field of operations by cutting them away, or strip- 

 ping off the leaves where it is important to save the branch. 



With respect to the above classification of the Ai)hides in accordance 

 with their habits and modes of life, it is proper to remark that it must 

 be regarded as one of practical convenience rather than of scientific ac- 

 ciu-acy. This follows, from the fact that we sometimes find in the same 

 genus, as in that of Eriosoma, species which vary greatly in this respect; 

 and in other cases, as in the gall-louse of the grape leaf, {Phylloxera 

 vitifolii,) it is now pretty well determined that the same species may 

 form and inhabit galls at one period of its life, and at another migrate 

 to the roots. 



