138 ERNEST A. BACK. 



generic and specific description respectively. Whenever it 

 was not possible to obtain material of species, translations 

 from the original description, were they in other than the 

 English tongue, have been used, or were such in English, 

 direct copies made; but in either case full credit given. In 

 many instances, such as the most excellent and reliable de- 

 scriptions of Baron Osten Sacken and Dr. S. W. Williston, the 

 writer has preferred, even when he has had an abundance of 

 material before him, to use the original description in quota- 

 tion marks rather than attempt the most difficult task of 

 improving them; such, for example, as Baron Osten Sacken's 

 description of many Cyrtopogon species, specimens of which 

 have not been taken since the original descriptions were made. 

 No description, however, of the type material of either Loew or 

 Osten Sacken, and of most of that of Williston, has been used 

 until it had been previously compared with the type itself, a fact 

 which should make the description more reliable. 



In the preparation of this paper, the valuable Loew and 

 Osten Sacken type collections, the translations above referred 

 to, the most praiseworthy work of Dr. S. W. Williston, and 

 Aldrich's Catalogue have proved indispensable. Up to this 

 time almost no illustrations of North American Dasypogoninae 

 have been published except the line drawings of Dr. Williston 

 in the Transactions American Entomological Society of Phila- 

 delphia, and a half-tone plate by Dr. L. O. Howard in his 

 Insect Book, Plate XIX. In this paper, each genus, with few 

 exceptions, has been illustrated by an enlarged photograph 

 of one or more species, and it is hoped that these will be of 

 no small aid in the identification of genera. 



The division of the family into four subfamilies, Leptogas- 

 trinse, Dasypogoninae, Laphrinae, and Asilinas has been re- 

 tained. The table of genera is based upon that of Dr. Willis- 

 ton in the first edition of his Manual of North American Dip- 

 tera, but has been revised in several ways. It is hoped that 

 the reference in the table to the illustrations of the different 

 genera will be of assistance. Keys to the species of the genera 

 have been added whenever it has been feasible. Some trouble 

 will be experienced in the identification of the species of 



