VIII 



APHIDAE 



587 



producing unisexual (female) individuals, and a portion of those 

 that emigrated to the Litri.r remained thereon, producing also uni- 

 sexual (female) individuals, while the others returned to the Pi<-ri 

 and produced a sexual generation. How long the production of 

 the unisexual generations may continue has not been determined. 



Plii/llnj-i'ra. The PhyUnj<-,-<i, that has caused such an enor- 

 mous amount of damage in the Old World during the last thirty 

 years, is a small Aphid that was introduced from Xorth America 

 into Europe. In Xorth America it is not so injurious as it is 

 in Europe, owing, no doubt, to slight distinctions in the condi- 

 tions of life in the tsvo hemispheres, as one of which may be 

 mentioned that in Europe a larger proportion of the individuals 

 produced appear to confine themselves to feeding on the roots, 7'. 

 nixtntrix being one of the species that lives both in galls on 

 leaves, and underground on the roots. The species is one that 

 exhibits in their most complex form the peculiar phenomena of 

 Aphid life we have already mentioned. It has probably only 

 one congener, Phylloxera quercus, and of this Lichtenstein says 

 that in its cycle, from the starting-point of the winter-egg to 

 the assumption of the sexual condition, 

 it exhibits a series of no less than 

 twenty-one forms. 1 The life of Phyl- 

 loxera riixtnti'ix apparently differs essen- 

 tially from wliat we have described in 

 Cliri-iiirx, inasmuch as the migrations are 

 only between leaf and root of the same 

 plant the vine and not from one 

 species of plant to another. Some autho- 

 rities treat Phylloxera and Chermes as a 

 separate family under the name of Thyl- 

 loxeridae. 



Gall*. Like Phylloxera, many species 



of Aphidae live partially, others wholly, FIR. 286. Gall, or false cone 



,,f Chermes (Metis. Cam 

 in galls that are produced by plants as 



the result of one or more Aphids inter- 

 fering with a delicate part of the plant 

 when it is in a youncj and orowino- 



i/ O 



state. The usual position of Aphid galls 



is on a leaf or leaf-stalk. But in the case of the genus Chermes, 



1 Etit. Zui.t. Stettin, xxxvi. 1S75, p. 368. 



cl. 



liri.lue. The small ti_ 

 to the left, is a section in:i<lr 

 at the level indicated by 

 the pointing line /', ami 

 shows the chambers r"iituiii- 

 ing young. 



