PREFACE. V31 



8vo.) is still the best work, exhibiting the dipterological system. 

 Iu order to obtain information on the progress which science has 

 made since Meigen's age, this work may be followed by the study 

 of Walker's Diptera Britannica. The excellent plates by Mr. 

 Westwood, and the systematic arrangement prepared for the most 

 part by Mr. Haliday, give to this work a value not shared by Mr. 

 Walker's other publications. Next to these I would name Mr. 

 Macquart's Dipteres Exotiques, a work which, notwithstanding the 

 errors in many of its figures and the carelessness of nearly all the 

 descriptions, affords a great deal of useful information about the 

 systematic arrangement of Diptera. 



After having acquired a general knowledge of the system by 

 the study of these three works (or, if not familiar with the German 

 language, of the two latter only), the best plan will be to con- 

 centrate special study on one, or, at the utmost, on a few families 

 of Diptera, and to consult the monographic papers relative to 

 them, which are not difficult to procure, in order to obtain a com- 

 plete and sure knowledge of characters within a more limited field. 

 For even the smallest field will always be found wide enough to 

 afford opportunities for the most interesting discoveries. This 

 mode of obtaining a knowledge of the system capable of serving 

 as a solid foundation to valuable publications is certainly a long 

 and tedious one. It may be considerably facilitated, however, by 

 the use of a well determined collection of typical specimens of all 

 the families and genera, and it will afford me much pleasure to ex- 

 tend all assistance in my power to those who may prefer the latter 

 course ; for both my wish to become better acquainted with the 

 Diptera of North America and their desire to study the system 

 might well coincide to supply the wants of both parties. 



I am always ready to send in exchange for well preserved N. A. 

 Diptera forwarded to me (address Mr. H. Loew, Meseritz, Prussia) 

 a reasonable equivalent in accurately named representatives of the 

 genera. I should probably be obliged, in most cases, to send only 

 European specimens, whereas, perhaps, it would seem more de- 

 sirable to have N. A. species. But the number of species occur- 

 ring in perfect identity both in Europe and North America is 

 so surprisingly large, and, besides, there are so many N. A. spe- 

 cies exceedingly resembling well-known European ones, that the 

 best foundation for the study of N. A. Diptera would be a com- 

 plete collection of European species. It will be very useful to 



