4 6 NORTH AMERICAN DIPTERA. 



but that the number and arrangement of bristles in most 

 cases offer important, in some eases perhaps the only, 

 available criteria in generic classification. For that rea- 

 son it behooves the student to become familiar with the 

 subject, and to pay especial attention to their use in 

 many, if not most of the groups. 



Cephalic bristles. 



Vertical, two pairs, inner and outer, inserted more or 

 less behind the upper and inner corner of the eye, erect 

 or the inner pair convergent, the outer divergent; the 

 outer pair is usually the smaller and more frequently 

 absent. 



Postvertical, in the middle, near or back of the vertex, 

 convergent, erect or divergent; usually small, and often 

 absent. 



Ocellar, situated close to the ocelli, usually the foremost 

 ocellus; usually directed forward; often minute or absent. 



Frontal, a row of bristles on either side of the median 

 vitta descending often to or below the root of the anten- 

 nae on the side of the face anteriorly; of much use in the 

 classification of the Calypterae. 



Fronto-orbital, a bristle or bristles on each side of the 

 front near the orbit, immediately below the vertical bris- 

 tles; there may be one, two, or none on each side. 



Lower fronto-orbital , situated on the lower part of the 

 front near the eyes, and not quite in line with the fronto- 

 orbitals. They are not of frequent occurrence. 



Cruciate, a pair of bristles in the middle of the lower 

 part of the front, directed inwardly and forwardly ; ob- 

 served in some Calypterae and Acalypterae. 



Vibrissa, a stout bristle situated near or a little above 

 the oral margin, near the front edge of the mouth open- 

 ing; of important use in the classification of the Acalyp- 

 terae. 



Facial, a scries of bristles on either side of the middle 



