1^ 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 sum- 

 mer — until at least the third or fourth has been reached before the 

 winged form (/3) 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 a^ 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 tliemselves, 

 between galloecola and radicicola are more apparent than real. In- 

 dividuals 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 larvae, 

 which are normally granulated with papillae, 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 {JVematus ventricosus Klug), 

 are often completely cast off at a moult. In Phylloxera they are very 

 variable in size, as we shall see, in Pileyi j and in quercus^ according to 

 several reliable authors, the tubercles which are characteristic of the spe- 

 cies in Southern France are entirely wanting around Paris. If we care- 

 fully study them in vastatrix we shall find that they consist of points 

 where the granulated skin is gathered around a fleshy hair in little ru- 

 gosities, and becomes darker (Fig. 4, ^). They do not occur in the 

 newly-hatched larva, are not visible immediately after each moult, and 

 are lost again in the winged individuals. In the form gallmcola 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 tu- 

 bercles 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 Mileyi^ to 

 be described. The ventral characteristics of the two types are iden- 

 tical. 



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 repeat- 

 edly 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 



