Page:Popular Science Monthly Volume 5.djvu/24

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14
THE POPULAR SCIENCE MONTHLY.

At this season of the year, with the exuberant juices of the plant, the swellings on the roots are large and succulent and the lice plump to repletion. One generation of the mother form (a) follows another—fertility increasing with the increasing heat and luxuriance of summer—until at least the third or fourth has been reached before the winged form (β) makes its appearance in the latter part of June or early in July.

Such are the main features which the development of the insect presents to one who has studied it in the field as well as in the closet.

This polymorphism, which at first strikes us as singular, is quite common among plant-lice, and many curious instances of still more striking character might be given. Even the differences themselves, between gallæcola and radicicola are more apparent than real. Individuals of the latter are often met with, which, in the comparative obsoleteness of their tubercles, are almost undistinguishable from the former; and the tubercles, like many other purely dermal appurtenances, are of an evanescent and unimportant character. Many insect larvæ, which are normally granulated with papillæ, not unfrequently have these more or less obsolete, and at some stages of growth have the skin absolutely smooth. The same thing holds true of tubercles, which, as in the case of the Imported Currant-worm (Nematus ventricosus Klug), are often completely cast off at a moult. In Phylloxera they are very variable in size, as we shall see, in Rileyi; and in quercus, according to several reliable authors, the tubercles which are characteristic of the species in Southern France are entirely wanting around Paris. If we carefully study them in vastatrix we shall find that they consist of points where thrugosities, and becomes darker (Fig. 4, i). They do not e granulated skin is gathered around a fleshy hair in little occur in the newly-hatched larva, are not visible immediately after each moult, and are lost again in the winged individuals. In the form gallæcola we shall find, upon careful examination, especially of the exuvia, that, as Max-Cornu has shown, there are rows of these short hairs, scarcely extending beyond the natural granulations and corresponding to those on the tubercles of radicicola. These hairs are more visible on the younger and smoother lice, after the first moult; and they are sometimes so stout, particularly on the abdomen, as to remind one of those on Rileyi, to be described. The ventral characteristics of the two types are identical.

Since I proved, in 1870, the absolute identity of these two types by showing that the gall-lice become root-lice, the fact has been repeatedly substantiated by different observers. Yet, strange to say, no one has heretofore succeeded in making gall-lice of the young hatched on the roots, though I formerly supposed that Signoret had done so. It is, therefore, with much satisfaction that I record the fact of having succeeded this winter in obtaining galls on a young Clinton vine from young radicicola, and of thus establishing beyond peradventure the