June, '08] JOURNAL OF ECONOMIC ENTOMOLOGY 179 



in a few instances. Prof. Sanderson* has already recognized it as a 

 good species. We have not taken this louse in Colorado, but speci- 

 mens that were sent me by Mr. J. J. Davis, State University, Urbana, 

 Illinois, have been examined and prove to be easily distinguishable 

 from any of the other species mentioned in this paper. In about 

 three fourths of the examples examined, all of which were apterous 

 females, the third and fourth joints of the antenna were united in one 

 with no signs of a dividing suture. Dr. "Weed in his description of 

 this species describes it as having six-jointed antennae. In the speci- 

 mens having joints 3 and 4 separate, joint 3 but slightly exceeded 

 joint 4. and the two joints together made one sub-equal in length with 

 joint 7. In gossypii joint 3 alone is always longer than joint 7. A. 

 forhesi is also smaller and is a root feeder. 



Influence of Host Plant upon Aphid Characteristics 



An impression seems to be more or less prevalent that a species of 

 plant louse may vary much in structure and general appearance, de- 

 pending upon the plant upon which it lives and draws its nourish- 

 ment. 



In all our experience transferring lice from one food plant to an- 

 other and observing them upon widely varying plants in a state of 

 nature or in hot-houses, we have never had any reason to think that a 

 species is perceptibly changed in appearance because of a change of 

 food plant. I am aware that there are migrating forms that are dif- 

 ferent in appearance from their immediate ancestors and that they 

 may go to a different food plant, but in such eases the change came 

 before migration or was "predestined" to appear in the first genera- 

 tion after migration. For example, the stem mother of Phorodon 

 humuli upon the plum is quite different from the migrant that goes 

 back to the hop^. but the change came before deserting the plum. The 

 oviparous female in the fall, which is the product of the return mi- 

 grant from the hop. is very different from the migrant, but not at 

 all because of its change in diet. It is the sexual female form of the 

 species and what it develops into was determined in every case before 

 the parent left the hop. Such changes as these, coincident with a 

 change of food plant in the life histories of plant lice, can be dupli- 

 cated many times over by instances where there is not a change of 

 host plant. As familiar illustrations recall the remarkable variations 



4Bull. 49, Del. Agr. Sta. 



H do not mean by this that the viviparous females upon the plum can be 

 distinguished from the viviparous females upon the hop. 



