HEMIPTERA. 



161 



of the leaf, and containing a wingless agamic plant-louse and 

 her eggs ; second, when the fibrous roots of a sickly vine are 

 examined, we find, if the disease is due to this insect, that the 

 minute fibres have become swollen and knotty; or, if the disease 

 is far advanced, they may be entirely decayed. Upon these root- 

 swellings we also find an agamic, wingless, egg-laying plant-louse, 

 the author of the mischief. 



The insects found upon the roots differ slightly from those found 

 within the galls ; but their specific identity is now generally accept- 

 ed. A careful study of this insect has revealed still other forms. 

 So that now we can say that the species is presented to us under 

 the three distinct forms described below. 



Of the first form there are two types : one, the root-inhabiting 

 type, which causes the knots on the roots ; second, the gall-inhabit- 

 ing type, which produces the galls upon the leaves. The gall-inhab- 

 iting type is simply a dimorphic form, which does not constitute an 

 essential part in the cycle of changes through which the species must 

 pass. It only appears when the insect infests certain species of grapes. 



The following epitomized account of the life-history of this spe- 

 cies is condensed from Dr. C. V. Riley's Sixth and Seventh Mis- 

 souri Entomological Reports: 



The Grape Phylloxera hibernates upon the roots of the grape, 

 mostly as a young larva of the first or sedentary, agamic, wingless 

 form (Fig. 135). With the renewal of vine-growth in the spring, 



