ADVERTISEMENT. 



TriE present publication is the first part of a work on North 

 American Diptera in process of preparation by Dr. H. Loew, of 

 Meseritz, Prussia (one of the highest living authorities on the 

 subject), undertaken at the especial request of the Smithsonian 

 Institution. The materials have been derived principally from 

 the collection of Baron R. Osten Sacken, of the Russian Legation 

 in Washington, kindly intrusted to the author for examination. 



As explained by Dr. Loew, the work will appear in monographs 

 of genera and families, sufficient materials being at hand for illus- 

 trating particular groups only, without relation to their systematic 

 sequence. 



The Institution is under obligations to Baron Osten Sacken for 

 editing the work, adding species described by Dr. Loew subse- 

 quent to the reception of his manuscript, and for correcting the 

 proofs. He has also added a monograph of the Cecidomyidce, a 

 group of much interest, and one to which it was considered of 

 importance to call the early attention of investigators. 



JOSEPH HENRY, 



Secretary S. I. 

 Smithsonian Institution, 



Washington, March, 1S62. 



ACCEPTED FOR PUBLICATION, JULY, 1SG1 



PHILADELPHIA: 

 COLLINS, PRINTER. 



