INTRODUCTION. 



THE order of two-winged insects, known as flies or Diptera, 

 includes nearly forty thousand known species from different 

 regions of the world. Since many of the species are very small 

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

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

 there certainly remain very many more to be yet made known. 

 From North America not far from four thousand species have 

 been studied, and we probably have as many more awaiting 

 discovery. Our knowledge of the dipterological fauna has 

 progressed with increased rapidity during the past ten or 

 twenty years, but vast fields for profitable study yet remain 

 open for the serious investigator. In North America the 

 results to be obtained are almost inexhaustible. Nearly every 

 family yet awaits the conscientious monographer. The des- 

 cription of new species is the much less interesting of the 

 work to be done, and perhaps the less profitable. At the 

 present time the rapidly increasing number of short papers 

 descriptive of new forms is rendering the determination of 

 species more and more difficult. 



To the student beginning the study of this interesting group 

 of insects, some words of advice or caution may 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 surest 

 dissipated by attempting to refer it to some species. Until the 

 student has acquired a sort of intuitive acquaintance with the 

 different families the work may be somewhat tedious, but by 



