TACHINIDJE DEXIIDJE. 145 



is always narrowed or closed(save in those rare cases in which 

 the distal section of the fourth is obliterated); auxiliary vein 

 distinct in its whole length. Tegulae large. 



No other group of flies has presented so many difficulties 

 to the student of systematic dipterology as has the present 

 one. In its entirety, with but few exceptions, it is easily 

 enough differentiated. Flies in which the arista is bare, the 

 tegulae are well developed, and the first posterior cell is nar- 

 rowed or closed, maybe unhesitatingly referred to the Tachin- 

 idse. The Dexiidae gradually merge into that family, and 

 it is not always easy to positively distinguish them. The 

 presence, however, of aristal plumosity with the other charac- 

 ters of the more typical Tachinids, that is, those in which the 

 dorsuin of the abdomen is distinctly bristly, will remove 

 doubt of the correct location of any species possessing such 

 characters. Unfortunately the differentiation of the genera 

 and species is vastly more difficult, and will require much pa- 

 tient and exhaustive study before a satisfactory solution is 

 reached. About two hundred and seventy-five genera, or 

 one-fifth of all the dipterological fauna, have already been ac- 

 credited to North America. Some of the names proposed 

 will unquestionably be reduced to synonyms, but not many, 

 as the characters used for generic distinctions are often exceed- 

 ingly slight. So inconspicuous are the characters in many 

 species that only the practiced eye will detect them. 



I have not attempted to define or tabulate the genera. To 

 do so, even imperfectly, would require far more time than is 

 at my command. The best that I can do now is to give a fair- 

 ly complete list of the genera said to occur in North America, 

 with their bibliograpical references. For further study, the 

 student is referred to the works of Brauer and Bergenstamin, 

 the various papers of Townsend and Coquillett, and the vol- 

 ume of the Biologia Centrali-Americana by van der Wulp. 



The habits of the mature fly are similar for nearly all the 

 members of the group. They will be found on vegetation, on 



