IV PEEFACE. 



by the Directors of the Hofnaturalien Kabinet in Vienna, afforded 

 me information of the most valuable character. 



Although the materials thus put at my disposal can be called 

 abundant, they did not by far reach the extent which I could 

 desire for the preparation of such a monograph. I hope, how- 

 ever, that new supplies will enable me soon to attempt a renewed 

 and more thorough work on the same subject. I have to thank 

 most cordially all those who have contributed by their collections 

 towards the completion of this monograph, and at the same time 

 I earnestly request all North American collectors who take any 

 interest in the order of Diptera to favor me by similar communica- 

 tions. They can reach me either through the medium of the 

 Smithsonian Institution or through Baron Osten-Sacken. 



"With regard to the systematic distribution, I have continued to 

 build upon the foundation first laid by Mr. Haliday in his admira- 

 ble paper on Dolichopodidae, contained in Walker's Diptera 

 Britannica, and later developed by me in the seventh part of my 

 " Neue Eeiiraege.' 1 ' 1 To the genera, adopted and denned in those 

 papers, I have added afterwards the genera Plagioneurus and 

 Lyroneurus, based upon North American species. (Conf. Wien. 

 Entom. Monatsschr. 1857, p. 37.) In the eighth fascicle of my 

 "Neue Beitraege," which was a prodrome to the present work, I 

 have further added the genera Pelastoneurus and Diostracus ; the 

 first embraces some species which formed previously a sharply 

 limited and well characterized group within the genus Gymnop- 

 ternus ; the latter was established for a species which, by its 

 general habitus, reminds equally of Thinophilus and Aphrosylus, 

 but is distinguished from both by the presence of distinct hairs 

 on the upper side of the first joint of the antennae. In the 

 present publication I have added the new genus Paraclius, 

 which combines the neuration of the wings of Pelastoneurus 

 with some characters of Gymnopternus and at the same time is 

 too distinct from the species of the latter genus, to remain united 

 with it. 



Thus the number of genera, the usefulness or necessity of which 

 I at present recognize, amounts to forty-three. After a general 

 introduction, I give a table for their determination, and then cha- 

 racterize them in detail at the proper place. It will hardly be 

 necessary for me here to attempt to correct the misapprehension 



