563 DIPTERA. 



They live on the juices of plants, and undergo their trans- 

 formations either in these plants, or in the ground. 



The transformations of these insects offer some peculiari- 

 ties that do not seem to have been described by European 

 naturalists, and probably are not well understood by them. 

 Three modifications in the process have been observed in 

 this country, and examples of these are affordeed by Cecido- 

 myia Salicis, destructor, and Tritici. In all of them the pupa 

 has the limbs and wings free or unconfined, and becomes 

 active shortly before its final change, being enabled to crawl 

 out of the place where it had hitherto lodged, when about 

 to take the winged form. It appears also that these Ceci- 

 domyians retain the larva-skin when the insect is changed 

 to a pupa ; this skin undergoing only certain alterations in 

 the course of the process, without being thrown off. The 

 abdominal part of the larva-skin remains with little or no 

 change ; the fore part of the body becomes swollen, shining, 

 and apparently gelatinous, and allows the budding limbs and 

 wings of the pupa to push outwards, each carrying with it an 

 enveloping portion of the skin, which by extension or growth, 

 or by both, is modified so as to suit the changed condition 

 of the insect. This peculiarity was first made known to me 

 by a letter from Dr. Asa Fitch, of Salem, New York, who 

 has paid much attention to the natural history of the Ceci- 

 domyians, and has published several elaborate essays upon 

 them in " The American Quarterly Journal of Agriculture 

 and Science," and in " The Transactions of the New York 

 State Agricultural Society." In these essays, however, the 

 point under consideration is not so distinctly stated and de- 

 scribed as in his letter. I am also indebted to him for galls, 

 containing larvae of the willow gall-fly. These, with speci- 

 mens of the Hessian fly in the flax-seed state, received from 

 him and from other correspondents, have enabled me to 

 verify the result of his observations. 



The willow gall-gnat, or gall-fly, is one of the largest of 

 our species. It has been described and figured by Dr. Fitch, 



