2 DIPTERA OF NORTH AMERICA. 



to its plan could not, afford a survey of the North American 

 Fauna Dipterologica, corresponding to the present state of sys- 

 tematic Dipterology; on the contrary, sketching such a survey is 

 one of the tasks to which it looks forward to as one of the first 

 fruits of its publication. It would be quite impossible to draw 

 such a systematic survey of the hitherto known North American 

 Diptera from the Catalogue itself, since it comprises the publica- 

 tions of the authors of different times and countries, of writers who 

 had the most different systematic ideas and points of view, and 

 since, in consequence of its plan, it could not but include such 

 papers as are devoid of any solid knowledge of systematic Dipter- 

 ology to which, above all, the descriptions of Rob. Desvoidy, 

 and, in a still higher degree, those of Mr. Walker belong. Conse- 

 quently a survey of those families and genera which North America 

 really possesses, is to be acquired in no other way than from the in- 

 spection and careful investigation of the species themselves. The 

 rich collections of Baron Osten-Sacken have enabled me to examine 

 a number of North American species sufficient to allow me to ven- 

 ture an essay of the kind indicated. In this survey I have adopted 

 for the North American Fauna the same area as that of Baron 

 Osten Sacken's Catalogue, the materials upon which I establish my 

 work corresponding to this area. Still it cannot be denied, as far 

 as I am able to judge, that this area, in its southern extent, reaches 

 beyond the limits of the North American zoological province. In 

 order to give a true, though of course not complete sketch of the 

 North American Dipterological Fauna, I can, besides such species 

 as I know by my own inspection, have regard only to those the 

 systematic location of which is in no way doubtful. 



For many years past all Diptera have been divided into two 

 large sections, Nemocera and Brachycera. In the Diptera of the 

 first section the antenna, having the fundamental form of a thread, 

 consists of many joints, two of them being called the joints of the 

 scopus, the following those of i\\Q flagellum. The latter are all of 

 the same structure, although this structure varies in different spe- 

 cies. The first joint of the flagellurn, i. e., the third of the whole 

 antenna, is never so distinguished in size or structure that one 

 might consider the succeeding joints as its accessorial appendages, 

 nor is the connection of the joints (with the exception of the Bibi- 

 onidce, Mycetophilida:, and a few others) such as might lead us to 



