No. 1.] EILEY AND MONELL ON APHIDIDJ]:. 3 



open oue. Tliey instinctively seek tlie bark of the tree, and there give 

 birth to the sexual individuals, either directly or through intervening 

 generations. 



It is my purpose in a subsequent paper to go more fully into a con- 

 sideration of the habits and classification of this interesting family of 

 riant-lice; but my present object will best be accomplished by giving a 

 full account of the two commoner gall-making species found upon the 

 American Elm, with less complete accounts of some other species. It 

 will be seen by the facts recorded, and by the descriptions, how futile all 

 attempts must be to establish anything like a natural system of classifi 

 cation, whether the nmnber of antennal joints, the character of wiug- 

 venation, or the habits be considered; and the lesson I would draw from 

 the study of these minute insects is the same that must be drawn by all 

 naturalists who thoroughly study any one group, viz., that any system 

 of classification will be unsatisfactory, except on the hypothesis that it 

 is purely a matter of convenience. We find extreme variation in the 

 number and i^roportions of antennal joints in the different stages of the 

 same species. We find a great tendency to variation even in the ptero- 

 gostic characters ; and, finally, there is not even unity of habit in the 

 species of the same genus. The deflexed or horizontal i^osition of the 

 wings has no value in this sub-family, because most of ihe species carry 

 their wings horizontal while yet in their galls or for some time after issu- 

 ing from the pupa. That all of the insects of this sub-family possess, 

 however, at least one featiu?e in common with the species of Phylloxera 

 so far known, namely, the mouthiess, wingless, and generally degrada- 

 tional nature of the sexual individuals, and the production by the female 

 of a solitary, impregnated egg, there can be little donbt, since I have 

 traced these sexual individuals and this impregnated egg, not only in 

 the cases indicated in this paper, but further in Scliizoneura Bileyi (Thos.) 

 and Scliizoneura lanigera (Hausm.) ; while M. J. Lichtenstein, as ah-ead^- 

 stated, has discovered them hi several of the European species of this 

 sub-family. Another feature common to the whole sub-family, though by 

 no means pecuhar to it, is the flocculent exudation from the body and 

 the absence of bright coloration, the winged females having, all of them, 

 a dull, dark ground-color of head and thorax. The front wings have 

 invariably a fold or thickening of the posterior marginal vein in the 

 region of the first discoidal ; and the hind wings are correspondingly 

 produced on the costal margin and armed with booklets that catch hi 

 the fold in flight. These are features common to all Apliitlidw known 

 to me ; but their prominence or faintness often has specific value, and 

 the angle on the hind wmg may, for descriptive purposes, be called the 

 " hook-angle." 



