INTRODUCTION. XIII 



Front. The space between the eyes in all dichoptic flies, 

 limited by the upper margin of the head and the line drawn 

 through the root of the antennae is called the front. It may 

 be wide or narrow, excavated or convex, etc. 



Vertex. The uppermost part of the front, near the margin 

 of the occiput, which is here called the vertical margin. 



Vertical triangle. The triangle at the upper part of the 

 head, between the eyes in holoptic flies. It bears the ocelli, 

 which when placed on a triangle indicated by grooves or de- 

 pressions is called the ocellar triangle. 



Frontal triangle. In holoptic flies, the triangle between 

 the eyes and the root of the antennae, the apex of which is 

 above. 



Ptilinnm. In the Cyclorrhapha an inflatable organ capable 

 of being thrust out through the frontal suture just above the 

 root of the antennae, and which is used by the imago in spring- 

 ing off the cap to the puparium when about to extricate itself. 



Frontal lunule. An oval or crescentic space just above the 

 root of the antennas in Cyclorrhaphous flies, bounded by the 

 frontal suture. 



Epistoma or Peristoma. The oral margin and an indefinite 

 space immediately contiguous thereto. 



Antennal fovea. A groove or grooves in the middle of the 

 face, as though for the lodgment of the antennae, bounded on 

 the sides by the facial ridges. 



Cheeks. The space back of the face and below the eyes. 



Orbits. The space immediately contiguous to the eyes, 

 sometimes indicated by structural characters, at other times 

 indefinite. It is called facial, frontal, etc., from the position. 



Clypeus. A part of the mouth structure, often visible be- 

 low the margin of the mouth in front as a more or less visor- 

 shaped piece. 



MOUTH-PARTS. 



The mouth-parts of diptera are wholly suctorial. They 

 differ not a little in the different flies, as might be supposed 



