PREFACE. IX 



North American diptera now extant, it is always interesting, in a 

 list, like mine, to find its catalogue. 



In the present catalogue, I occasionally refer to Say's manu- 

 script notes. These notes, in Say's own handwriting, I found in a 

 copy of Wiedemannls aussereuropaische Zweiflugler, which belonged 

 to Say, and is still preserved in the Library of the Academy of 

 Natural Sciences in Philadelphia. 



The compilation of this catalogue has been suggested and en- 

 couraged by the Smithsonian Institution ; and I earnestly hope 

 such encouragement, seconded by the assistance of all friends of 

 natural history, and collectors in all parts of the United States, 

 may further the progress of this branch of entomology as rapidly 

 as the study of the Coleoptera of this country was furthered under 

 similar auspices! 



On the opposite page will be found a few brief directions for 

 catching and preserving specimens of diptera. 



R. OSTEN SACKEN. 



Washington, D. C, March, 1857. 



