INTEODUCTIOK 



The order of two winged insects, known as flies or Diptera, 

 includes nearly forty thousand known species from different 

 regions of the world. As many of the species are very small, 

 often minute, and inconspicuous, and as the order has received 

 only a small share of the attention of collectors and students, 

 it is very probable that not one third of the real number in 

 existence have yet been made known. In North America not 

 far from four thousand species have been studied, and we 

 doubtless have as many more awaiting discovery. The knowl- 

 edge of the dipterological fauna has progressed with increas- 

 ing rapidity during the past ten or twenty years, but vast 

 fields for profitable study yet remain open for the serious in- 

 vestigator. In Korth America the results to be obtained are 

 almost inexhaustible. Nearly every family remains for the 

 conscientious monographer. The mere making of new species 

 is the least interesting of the w^ork to be done, and perhaps 

 the least profitable. At the same time the increasing litera- 

 ture makes the determination of desultory species more and 

 more difficult. 



To the amateur, beginning the study of this interesting 

 order, some words of advice or explanation will be of service. 

 The present work can make no pretensions to completeness 

 in the characterization of genera, and he should never depend 

 upon mere tables in the absence of other information. Doubt 

 of the right generic location of a specimen may often be siirest 

 dissipated by attempting to refer to some species. Until the 

 stiident has acquired a sort of intuitive acquaintance with the 

 different families the work may be somewliat tedi(ms, but by 



