1864.] 545 



instance, noticed a willow-bush bearing apparently numerous specimens 

 of both S. brassicnides and S. sti-ohi'loides, but on examining the foliage 

 I have always found, that the two different willows that bear these two 

 galls were here growing promiscuously from the same spot of ground, and 

 that each branch of each species bore its appropriate gall, and never the 

 gall peculiar to the other species of willow. The instances where these two 

 willows grew side by side, or only removed a short distance from each 

 other, and where I found each bearing exclusively its appropriate gall, are 

 almost innumerable. This feet is the more remarkable, because the Wil- 

 lows form a very extensive genus, with the species often separated from 

 each other by very minute distinctions. We meet, however, with an analo- 

 gous case in the gall-making Hymenopterous genus Oj/mps, where with 

 occasional exceptions each species is confined to a distinct species of 

 Oak ; while, on the other hand, the gall-making Cecidomj/ia of the 

 Hickory are said by Osten Sacken to be " found indifferently on the 

 various species of that tree." {Synopsis Dipt. M. A., p. 191.) 



It does not follow, however, bec;mse certain galls are found exclu- 

 sively on particular species of willow near Rock Island, that the iden- 

 tical same gall may not occur in other localities on other species of 

 willow which do not grow near Rock Island. A willow-gall (SaUcis 

 Fitch, which being preoccupied has been changed by Osten Sacken to 

 rigid se) closely resembling, so far as can be judged from Dr. Harris's 

 brief description, my S. siliqua, which is found on Salix humilis Mar- 

 shall, is said by Dr. Fitch to be found on S. rigida and S. lucida; and 

 I have found a gall which differs only in some few slight characters 

 from that found on S. humilis, and which for the present I consider as 

 identical with it, to occur sparingly on S. cordata Muhl.; and though 

 I could not succeed in breeding the imago from this gall, yet the larvae 

 of the two galls were absolutely undistinguishable. S. rigida, one of 

 the two willows on which Dr. Fitch found his gall, is regarded now by 

 most botanists, according to Mr. Bebb, as a mere variety of S. cordata 

 on which I found one of my two galls. I have also found a single spe- 

 cimen of what for the present I regard as the same gall on S. discolor, 

 So that if the four galls be in reality identical, we have here a case of 

 the same gall growing on four distinct species of willow, S. rigida 

 ( = S. cordata), S. lucida, S. humilis and S. discolor. 



The species of willow which grow near Rock Island, all of them in 



