ADVERTISEMENT. 



The present publication is the fourth part of a work on the 

 Diptera of North America. It has been prepared at the request 

 of the Smithsonian Institution by Baron R. Osten Sacken, and 

 is based almost exclusively on his own collections. 



Parts I and II of the series were written by Dr. H. Loew, of 

 Meseritz, Prussia, principally from the examination of specimens 

 furnished by Baron Osten Sacken. Part III, also by Dr. Loew, 

 is in an advanced state of preparation. The work is published 

 in successive monographs of families and genera, when sufficient 

 material is on hand for illustrating particular groups, without 

 reference to systematic seqifence. 



JOSEPH HENRY, 

 Secretary S. I. 



Smithsonian Institution, 



Washington, December, 1868. 



PHILADELPHIA: 

 COLLINS, PRINTER. 



