568 
DIPTEKA. 
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, 
