IV. 



ON THE NORTH AMERICAN EPHYDRINIDAE, 



The family of Ephydrinidte is taken here altogether in the extent 

 which was given to it by Stenhammar, the diligent monographer 

 of the Swedish species, and by Walker in his work on the British 

 Diptera, the latter founded on Haliday's valuable observations. 



The characters easily distinguishing the Ephydrinidte from all 

 the other families of Diptera acalyptera may be set down as fol- 

 lows: Face more or less, often considerably convex; either with- 

 out any impression at all beneath the antennae, or moderately im- 

 pressed, but never provided with membranous antennal furrows. 

 Antenna3 short, first joint small ; antennal bristle either nearly 

 bare, or pubescent, or pectinated on the upper side only. Oral 

 cavity rounded, in most of the genera of considerable size; cly- 

 peus distinct, in some genera retracted in the oral cavity, in the 

 remaining genera prominent over the oral margin, in some of them 

 of a rather large size; palpi small; mentum short, more or less in- 

 crassated. Thorax rather quadrangular; scutellum proportionately 

 large. Abdomen of very variable form, consisting of six seg- 

 ments in the males; the females have one short, and generally not 

 distinctly visible, segment more. The sixth segment being al- 

 ways small and generally much concealed under the fifth, the struc- 

 tural relations depend on the conformation of the five first seg- 

 ments; of these the first is often much shortened and sometimes 

 nearly connate with the second, a circumstance which has led 

 authors to omit it in the enumeration or to count the two first 

 segments for one; this is to be borne in mind in order to understand 

 their descriptions; in mine, I have always counted the first seg- 

 ment as distinct, however difficult it may be to observe it. The 

 fifth segment is also of very variable structure, generally nearly 

 equalling the foregoing in size, rarely considerably longer in both 

 sexes or in the males, still more seldom much smaller, in which 

 case the fourth segment, especially in the males, is longer than 

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