278 Beuj. D. Walsh on the Insects inhabiting the Galls 



and even of Coleopterous Pseudo-galls.* It will be contended perhaps 

 that I am arguing in a circle, and that when, as in the case of Baron 

 Osten Sacken's two Ct/nips, the galls are quite different and the insects 

 exactly alike, then I cousider the insects as distinct species ; and when 

 both the galls and the insects are exactly alike, then I consider the in- 

 sects as the same species, thus in effect assuming the existence of the 

 very criterion which I am attempting to prove. But there are no in- 

 termediate grades between these two cases to prove their similarity ; 

 which would inevitably take place if the criterion in question had no 

 real existence in Nature. Osten Sacken's two Oak-galls, for instance, 

 are so totally unlike each other internally, that out of a thousand spe- 

 cimens of each it would be impossible to find any two, that the most 

 ignorant person would be likely to confound ; and the same thing may 

 be said, with occasionally a few grains of allowance, of the other in- 

 stances adduced above. (§§ 1st and 2nd.) Whereas in the other class 

 of cases, where, in galls made by Gall-flies, Gall-guats, Plant-lice, and 

 Saw-flies, both the galls and the insects are alike, the galls that occur 

 on different species of the same genus of plants resemble one another so 

 closely, that, on the most attentive study of very numerous specimens, 

 no constant distinctive character whatever can be discovered. Nay, 

 it has even been found by Dr. Ratzeburg, as quoted by Osten Sacken, 

 that a European Gall-fly, Ct/nips fecundatrix, inhabiting normally a 

 European species of Oak, produced the very same kind of galls when 

 it attacked some American Oaks in his garden, that it produced on the 

 European Oak. (Proc. etc. I, p. 2-18.) 



But even if we tide over the difficulty, by assuming that all the 

 similar pairs of gall-makers producing distinct galls are identical, what 

 can we do with the 3 examples referred to above among the Saw-flies, 

 where the inquilinous species are apparently identical with gall-making 

 species ? (§§ 3rd — 5th.) Are we to believe that each of these 3 pairs of 

 so-called species are really identical, and that one and the same species 

 sometimes makes galls for itself, and sometimes inhabits a variety of 

 totally distinct galls made, not by Saw-flies, but by Gall-gnats? I 



dian, inhabiting undistinguishable galls on two distinct species of Rhus, will 

 be noticed below. 



|| Gall S. pomum n. sp. on Salix cordata and S. discolor, (the imago not reared 

 from the latter); S. ovum n. sp. on S. cordata and S. ovulum n. sp. on S. humilis. 

 (Most probably distinct species ; the imago not bred from the latter and the 

 larva? constantly of a different ground color.) 



* Pseudo-gall inornata n. sp. occurring both on Salix longifolia and on Popu- 

 lus angulata. 



