294 [Deoemrer 



On the genera of APHID^ found in the United States. 

 BY BEN.J. D. WALSH, M. A. 



The chief object of the following paper is to direct attention to the 

 various generic forms of the Aphis family, which I know to occur 

 within the limits of the United States either from observation or from 

 books. I do not possess Koch's great work on this family, and unlike our 

 more fortunate Eastern brethren, we Western naturalists have no Public 

 Scientific Libraries to aid us in our investigations. In order therefore 

 that no mistake may arise as to the generic limitation of species, and also 

 for the sake of brevity, I have compiled, partly from such resources as 

 are at my disposal and partly from my own investigations, the following 

 Synoptical Table of U. S. Grenera. Some of the old genera which are 

 retained are ignored by Koch, as I have been kindly informed by A. 

 Agassiz, Jr. Esq., who has obligingly forwarded to me such extracts from 

 Koch's hookas I asked of him; one genns (Thelaxes') has not hitherto 

 been discovered in the United States, and another ( Oalaj)his) is, so far as 

 I am aware, entirely new. Subjoined will be found references to all the 

 described U. S. species known to me, and brief descriptions of such as 

 appear to be new, always from the dried specimens except it is otherwise 

 stated, the food-plants being given whenever they are known. All the 

 new species occurred near Rock Island, Illinois. Imperfect as they are, 

 such descriptions may perhaps serve some useful purpose. 



Linnaeus long ago remarked on the difficulty of distinguishing the vari- 

 ous species of Aphidse. If we suppose, as some authors have done, that 

 similar species of Aphidse inhabiting distinct species of the same botanical 

 family are therefore necessarily distinct, the number of Aphidian species 

 will be enormously large. For example, a large and conspicuously mark- 

 ed red species described by Dr. Fitch as Aphis rudheckix occurs, accord- 

 ing to Dr. Fitch, on Rudbeckia laciniata, Solidago serotina and S. gigantea; 

 and a species differing only in some minute details of coloring, and which 

 I have little doubt is identical, occurs, as I have myself observed, on Sil- 

 phium perfoliatum and an undetermined species of Cirsium — all five of 

 the above plants belonging to the great Natural family Compositx. Here, 

 if difference of food-plant makes difference of species, we get from three 

 to five species of Aphis in the place of one. But I am myself acquainted 

 with many species, found on plants of distinct natural families, which are 

 either entirely undistinguishable when the living insects are placed side 



