DIPTERA. 451 



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 peculi- 

 arity 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 Cecidomyians, and has published 

 several elaborate essays upon them in " The American Quar- 

 terly Journal of Agriculture and Science," and in " The Tran- 

 sactions of the New York State Agricultural Society." In 

 these essays, however, the point under consideration is not so 

 distinctly stated and described as in his letter. I am also in- 

 debted to him for galls, containing larvae of the willow gall-fly. 

 These, with specimens 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, under 

 the name of Cecidomyia Salicis* On account of the size of 

 the larva and the ease with which it may be raised, it is an 

 exceUent object for the, observation of the transformation that 

 is peculiar to it and to other species of the genus. It inhabits 

 a small woody gall, growing at the ends of the slender twigs 

 of the American basket-willow [Salix rigida), and other dwarf 

 willows. This kind of gall is of an oval shape, about three 

 quarters of an inch long, by three eighths of an inch thick, 

 and is terminated by a brittle conical beak, which seems to me 

 to consist of the unexpanded and dry terminal bud of the twig. 

 Upon being cut open in the winter or spring, a longitudinal 

 channel will be found in the middle, extending from the apex 

 of the beak nearly to the base of the gall, and lined in the 

 upper part with a delicate silken web. Within this hollow is 

 lodged a single orange-colored maggot, about one fifth of an 

 inch long. In the spring this maggot takes the pupa form, 



* " American Quarterly Journal of Agriculture and Science," Vol. I., p. 263. 



