Vlll PREFACE. 



In a list like this, completeness is the principal merit ; the sys- 

 tematical arrangement is of but secondary importance. I have 

 retained generally the generic groups adopted by Meigen and 

 Wiedemann, avoiding the subdivisions introduced by modern au- 

 thors. In cases only, when a new genus had been especially 

 adopted for one or several new species, and had not been detached 

 from an older genus, I have retained it, supposing that its cha- 

 racters were of sufficient importance to justify its formation. In 

 the distribution of the Asilidce, Acroceridee, Stratiomydce and 

 Talanidce alone, I follow closely Walker's Supplement to the List 

 of Diptera of the British Museum, completing only its omissions. 



For the species within each genus, I have adopted the alpha- 

 betical order, so as to facilitate reference. Analytical tables for 

 determining the specific names, cannot be constructed from descrip- 

 tions only, without comparing the specimens themselves. Attempts 

 of this kind generally prove failures, and serve only to encourage 

 the fabrication of new species, by giving a false security to the 

 describer. 



In quoting Macquart's Dipteres Exotiques, I have always men- 

 tioned the volume and the page of the Memoires de Lille, in which 

 this work has been published, as well as the volume and the page 

 of the separate edition. The numbers of the plates and figures 

 are the same in both editions. 



Dr. Harris's Catalogue of Insects of Massachusetts, contains 

 many names of species which have never been described, or have 

 been so since, under other names, but I have considered it useful 

 to mention his names in my Catalogue. These species belong, 

 in part, to Mr. Harris's collection, which is still extant, and some 

 have been described lately by Mr. Walker, who retained Mr. Har- 

 ris's specific names. Others also may be found in collections in 

 this country, or abroad, labelled with Mr. Harris's names: the 

 wide-spread connections of this distinguished entomologist, making 

 this supposition very probable. I have thought these reasons a 

 sufficient excuse for a deviation from the original plan of a cata- 

 logue which was intended to comprise only described species. 



Although Walker's List of the Diptera of the British Museum 

 gives the description of the new species only, I have quoted all 

 the North American species mentioned in it, for the reason that as 

 this collection embraces probably the largest number of species of 



