Page:Popular Science Monthly Volume 5.djvu/14

4, the third or terminal much the longest, and by carrying its wings overlapping flat on the back instead of roof-fashion. It belongs to the whole-winged bugs (Homoptera), and osculates between two great families of that sub-order, the plant-lice (Aphididæ) on the one hand and the bark-lice (Coccidæ) on the other. In the one-jointed tarsus of the larva or newly-hatched louse, and in being always oviparous, it shows its affinities with the latter family; but in the two-jointed tarsus of the more mature individuals, and in all other characters, it is essentially aphididan. "In every department of natural history a species is occasionally found which forms the connecting link between two genera, rendering it doubtful under which genus it should properly be arranged. Under such circumstances the naturalist is obliged to ascertain by careful examination the various predominating characteristics, and finally place it under the genus to which it bears the closest affinity in all its details." So wrote Audubon and Bachman twenty-eight years ago; and what is true of genera is equally true of species, families, and of still higher groups. In the deepest sense all Nature is a whole, and all her multitudinous forms of animal and vegetal life are so closely interlinked, and graduate into each other so insensibly, that in founding divisions on too trivial differences we subvert the objects of classification. Thus, instead of founding a new family for this insect, as Dr. Shimer did, and as there seems a tendency on the part of others to do, it is both more consonant with previous custom, and more sensible in every way, to retain it among the Aphididæ.

—Not the least interesting features in the economy of our Phylloxera are the different phases or forms under which it presents itself. Among these forms are two constant types which have led many to suppose that we have to do with two species. The one type, which I have, for convenience, called gallæcola, lives in galls on the leaves; the other, which I have called radicicola on swellings of the roots. The subjoined table will assist to a clear understanding of what follows.

—The gall or excrescence produced by this insect is simply a fleshy swelling of the under side of the leaf, more or less wrinkled and hairy, with a corresponding depression of the upper side, the margin of the cup being fuzzy, and drawn together so as to form a fimbriated mouth. It is usually cup-shaped, but sometimes greatly elongated or purse-shaped (Fig. 2, a, b).