DIPTERA 



NORTH AMERICA. 



PART III. 



THE FAMILY ORTALIDi], 

 I. 



INTRODUCTION. 



In the variety of forms the Ortalidse are hardly surpassed by 

 any other family of diptera ; at the same time, they are hardly 

 equalled by any in the importance of the structural differences 

 occurring among them ; hence, they may be considered as one 

 of the most interesting families of the order. Nevertheless, 

 but little has been done as yet for the exact definition of the 

 limits of this family, as well as for its subdivision into smaller 

 groups. 



It would be impossible, therefore, to attempt a satisfactory 

 description of the North American species of Ortalidse, without 

 first settling the questions of the true limits of the family, of 

 its relationship to other families, and of the characters upon 

 which it is established. It would also be indispensable to 

 break the family up in subordinate groups and these groups in 

 genera. 



Of all these requirements, only one has been, as far as I can 

 see, fulfilled, and that is, the definition of the limits between 

 the Trypetidse and the Ortalidse, which I have tried to draw as 

 well as I could, in the first volume of these monographs and in 

 my earlier work on European Trypetidse. 



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