172 



mediately above the antennae. The families Lonchopteridae, Pipun- 

 culidae, and Platypezidae have a very poorly developed ptilinum or it 

 is undeveloped, but other characters appear to align them with Cyclor- 

 rhapha. 



In the Orthorrhapha, the imagines of the Nematocera have the 

 antennae usually much elongated and consisting of 7 or more distinct 

 joints; the Brachycera have the antennae consisting of 3 joints, the 

 third being either simple or composed of a number of closely fused 

 ring-like joints having the appearance of an elongated single joint 

 with more or less distinct subdivisions, and there is also, sometimes, 

 a terminal or dorsal arista or style. In the Cyclorrhapha the antenna 

 consists of 3 joints and a terminal or dorsal arista, the second joint in 

 some families being small and entirely or almost entirely enclosed 

 within the very large third, so that the antenna appears to consist of 

 only 2 joints. The wing veins offer a good character for the differ- 

 entiation of the families throughout the order, and being readily acces- 

 sible have been made much use of by taxonomists. The lower forms 

 have a much larger number of longitudinal veins than do the higher, 

 and this seems to point to a coincident reduction of antennal segmenta- 

 tion and wing venation in the evolution of the families of the order. 



An interesting piece of histological work might be undertaken in 

 connection with the development of the members of the Muscidae and 

 allied families, particularly in the observation of the developmental 

 processes of the head-parts, which are so much reduced in the larvae 

 of these forms. 



I present in this paper a key to the imagines of the families of 

 North American Diptera based upon the most recent data available. 

 I have made use of many characters that are not in existing text-books, 

 and in doing so I have simply attempted to put into writing data that 

 many specialists merely carry in their minds. The characters used by 

 Schiner and others for the separation of European families and genera 

 are as a general rule applicable to North American Diptera, but there 

 are many intermediate forms here that do not occur in Europe, and 

 as the keys in use here have largely been copied or adapted from those 

 in use in Europe considerable discretion is required in locating mem- 

 bers of many families by these keys. The described dipterous fauna 

 of this country is growing beyond the possibility of competent handling 

 by one individual, and frequently men who are authorities on certain 

 groups are unable to do more than make a guess at the family status 

 of a particular species. I make this explanation in the hope that stu- 

 dents will realize certain facts : that the beginner in the study is facing 

 a stiff task in this order; that the knowledge we possess concerning 



