P E E F A C E. 



The aim of this work requires no explanation. A complete 

 inventory of a branch of entomological science, at a given- 

 moment of its existence, is the best means for promoting its ad- 

 vancement. Nor does the imperfection of a publication of this 

 kind require an apology; any fair-minded reader is aware that 

 the chief merit to be expected is completeness, and that whenever 

 this is fairly attained, the usefulness of the work will far sur- 

 pass its shortcomings. It remains for me therefore, only to ex- 

 plain the rules that I have followed in preparing this Catalogue. 



Relation of the peesent catalogue to that of 1858. 

 The first Catalogue of North American Diptera, published by me 

 twenty years ago , was , and was meant to be , merely a com- 

 pilation of the existing literature on the subject. It brought 

 together a mass of references to the descriptions of about 1800 

 species, scattered in more than one hundred different works and 

 scientific papers. Although such a publication was an indispen- 

 sable preliminary step before any study of the North American 

 diptera could be attempted, it conveyed but a very vague idea 

 of the actual composition of the North American fauna of diptera. 

 It was impossible to ascertain, at that time, how many of the 

 specific names, enumerated in the Catalogue, actually represented 

 different species , and liow many were mere synonyms ; neither 

 was it possible to know, whether the species were placed in the 

 right genera, and even in the right families. In order to give 

 an idea of the extent to which this statement is true, I will 

 quote the genus Trypcfa, wliich (excluding the three species 

 named, but not described by T. W. Harris), contains forty-two 



