256 INJUKIOUS INSECTS 



the plant, the swellings on the roots are large and succu- 

 lent, 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 (b) makes its appearance in the latter 

 part of June or early in July. 



Since (in 1870) the absolute identity of these two types 

 was proved, by showing that the gall-lice become root- 

 lice, the fact has been repeatedly substantiated by dif- 

 ferent observers. (In 1873 galls were obtained on the 

 leaves of a Clinton vine from the root-inhabiting type, 

 thus establishing the identity of the two types). 



The moke manifest and external effects of 

 THE Phylloxera Disease.— The result which follows 

 the puncture of the root-louse is an abnormal swelling, 

 differing in form according to the particular part and 

 texture of the root. These swellings, which are generally 

 commenced at the tip of the rootlets, eventually rot, and 

 the lice forsake them and betake themselves to fresh ones 

 — the living tissue being necessary to the existence of this 

 as of all plant-lice. The decay affects the parts adjacent 

 to the swellings, and on the more fibrous roots cuts off 

 the supply of sap to all parts beyond. As these last de- 

 comi^ose, the lice congregate on the larger ones, until at 

 last the root system literally wastes away. The appear- 

 ance of the root fibres before and after they have been 

 attacked by the insect, is shown in figure 155, a. b, c. 



During the first year of the attack there are scarcely 

 any outward manifestations of disease, though the fibrous 

 roots, if examined, will be found covered with nodosities, 

 particularly in the latter part of the growing season. 

 The disease is then in its incipient stage. The second 

 year all these fibrous roots vanish, and the lice not only 

 prevent the formation of new ones, but, as just stated, 

 settle on the larger roots, which they injure, and which 



