AMERICAN DIPTERA. 145 



by a bloom upon the face, front and occiput. The face may 

 be bare except for bristles or hairs upon the oral margin, 

 called the mystax; or it may be densely pilose as in a species 

 of Cyrtopogon, and every gradation between these conditions 

 may be found. The clothing between the mystax and the 

 antennae is called the vibrissas. The front may be pilose as 

 in species of Holopogon, but more usually bare except for pile 

 or bristles along the orbits and upon the ocellar tubercle. The 

 occiput is variously clothed with bristles and hairs. The 

 bristles along the orbits on the occiput are called the occipito- 

 orbital bristles. The lower outer orbits of the eyes and the 

 base of the proboscis may be variously clothed with pile and 

 stiff hairs. 



Thorax (Plate II, fig. 2). — The mesothorax is by far the 

 most prominent part of the thorax, the pro- and metathorax 

 being greatly reduced. The prothorax is naturally divided 

 into two parts, the slender more or less elongated portion 

 which articulates with the head and which may be termed the 

 neck, and the larger posterior portion articulating with the 

 mesothorax which may be called the collar. The neck joins 

 the head at the center of the occiput which permits a very 

 free movement of the latter. The collar is distinctly separated 

 from the mesothorax behind by a suture, less distinctly so on 

 the sides; the anterior margin of the pronotum is usually 

 strongly elevated and well clothed with bristles or hairs. The 

 prothorax in such species as belong to the genera Holopogon 

 and Holcocephala is greatly reduced. 



The mesonotum occupies the greater part of the dorsum of 

 the thorax, and is usually well arched, reaching its greatest 

 development in this respect in Holcocephala. It extends back 

 to and includes the scutellum from which it is separated by 

 a suture. On either side, before the insertion of the wings is a 

 suture extending upward, being nearly or quite obsolete on the 

 dorsum, the transverse suture. The anterior superior angles 

 are called humeri; and the posterior superior angles, to the 

 side and above the scutellum, are called in this paper, the 

 posterior callosities. The mesonotum is separated from the 

 pleurae by the dorso-pleural suture. From before the insertion 



TRANS. AM. ENT. SOC, XXXV. (19) APRIL, 1909. 



