4 NORTH AMERICAN DII'TKRA. 



America, by Professor Aldrich, a very meritorious work, 

 has lessened materially the labor of its preparation, and 

 must add to its usefulness. It was with some misgivings, 

 however, that the author undertook the no inconsiderable 

 task, since his studies for some years past have been 

 almost wholly in a widely different field from that of 

 entomology vertebrate paleontology, a subject which, 

 indeed, has absorbed the larger share of his attention 

 for the past thirty years. 



But, he would not have undertaken the task, save 

 upon the kind assurance of assistance and contributions 

 by several of the leading students of diptera in the United 

 States. The chief progress during the last twelve years 

 has been in comparatively few families, the most of which 

 had been but indifferently well studied at the time of the 

 publication of the second edition; a progress in large part 

 due to the work of the present ^contributors. 



Professor C. F. Adams has kindly aided in a large 

 part of the work, especially the Cyclorrhapha, though 

 the writer assumes all responsibility for changes and ad- 

 ditions not directly accredited to the several contributors. 

 Professor Adams has also furnished the tables for the 

 very difficult families Dexiidae and Tachinidae, omitted 

 in the previous edition. As in the former edition, the 

 characters and table of the Dolichopodidae are wholly 

 the work of Professor Aldrich; he has also generously 

 assisted in other ways, and it need not be said that, with- 

 out the aid of his Catalogue, the labor of revision would 

 have been greater, and the results less satisfactory. 

 Mr. C. T. Brues, who has given so much critical attention 

 to the singular family Phoridse, has furnished the char- 

 acters and table of that family; one needs to compare his 

 table with that of the former edition to see how much has 

 been done in that group. Professor James Hine has re- 

 vised the characters and table of the Tabanidse. Professor 



