North American Diptera. 



INTRODUCTION. 



The order of two-winged insects known as flies or Dip- 

 tera includes more than forty thousand known species 

 from different regions of the world. Since many of the 

 species are small, or even minute, and inconspicuous, 

 and since the order as a whole has not received the at- 

 tention from collectors and students of entomology that 

 other and more attractive groups have, it is very certain 

 that many more await discovery. A very reasonable es- 

 timate would place the entire number of species of flies 

 at present in existence at more than eighty thousand. 

 From North America the recent catalogue of Aldrich 

 gives a list of about eight thousand species, distributed 

 in more than a thousand genera. The subject is a wide 

 one and replete with interest. 



To the student beginning the study of this interesting 

 order of insects a few words of advice or caution may not 

 be superfluous. The present work can make no preten- 

 sions to completeness in the characterization of genera, 

 at least in the majority of cases; that would require 

 a work many times larger than is the present one, and is 

 practically impossible at the present time. One must 

 not, therefore, depend entirely upon tables and figures 

 in the absence of other information and other assistance, 

 especially when he knows but few forms. If he does not 

 immediately succeed in securely locating his specimens 



