APHIDAE 



587 



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

 that emigrated to the Lario: remained thereon, producing also uni- 

 sexual (female) individuals, while the others returned to the Picea 

 and produced a sexual generation. How long the production of 

 the unisexual generations may continue has not been determined. 



Phylloxera. — The Phylloxera, 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 North America 

 into Europe. In North 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 two 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, P. 

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

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

 exhiljits 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 tlie starting-point of the winter-egg to 

 the assumption of the sexual condition, 

 it exhibits a series of no less than 

 twenty-one forms.^ The life of Phyl- 

 loxera vasfaJrix apparently differs essen- 

 tially from what we have described in 

 Chermes, 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 Phyl- 

 loxeridae. 



Galls. — Like Phylloxera, many species 

 of Aphidae live partially, others wholly, 

 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 young and growing- 

 state. The usual position of Aphid galls 

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



' Ent. Zeit. Stettin, xxxvi. 1875, p. 368. 



Fig. 286.— Gall, or false cone 

 of Chermes abietis. Cam- 

 bridge. The small figure, 

 to the left, is a section made 

 at the level indicated by 

 the pointing line a, and 

 shows the chambers contain- 

 ing young. 



