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MISC. PUBLICATION 6 31, U. S. DEPT. OF AGRICULTURE 



two regions, the clypeus (cl) and the epistoma (ep), the line of 

 separation between the two being a suture which is often obscure. 

 The clypeus may be flush with the rest of the face, or it may be sunken 

 underneath the antennae to form two antennal foveae, or it may be 

 completely sunken below the level of the facials; the middle of the 

 clypeus, especially when that region is sunken, may be elevated into 

 a median keel or carina (car). The epistoma often projects forward, 



Figure 1.- 



-Diagrammatic drawing of the head of a muscoid fly, from front view. 

 For explanation of abbreviations, see pages 17-19. 



although it may be level with the clypeus or may recede; it is bounded 

 below by the oral margin (om). 



In general the bristles of the head are named from the sclerites on 

 which they are located. An exception is the vibrissa (v), usually a 

 very strong bristle on each facial near the union of the clypeus and the 

 epistoma, or near the oral margin when the clypeus and epistoma are 

 not differentiated. The facials may bear, in addition to the vibrissae, 

 a series of short, ascending bristles, in one or more rows, extending part 

 way or fully to the bases of the antennae ; sometimes there may be one 



