22 NORTH AMERICAN DIPTKRA. 



tion, though the}'' may be somewhat revived temporarily 

 by the aid of moisture. 



Most flies have the eyes bare, or pubescent only when 

 seen under high magnification. Not rarely, however, 

 the \vhole or part of the eyes is covered with erect short 

 pile, a character which usually, perhaps always, finds its 

 greatest development in the male sex. The pubescence 

 or pilosity may be sparse or dense, short or long, and is 

 usually, though not always, of generic importance. 



Ocelli. On the upper part of the front in the middle, 

 between or a little back of the compound eyes, there are 

 three simple, small eye lenses, present in most diptera, 

 and called ocelli. The)' are by no means constant among 

 all the genera of some families, or even among all the 

 species of some genera. They are usually situated in 

 the form of a triangle with the apex in front ; sometimes 

 they are located in a nearly straight line transversely; 

 or, the middle one may be rarely absent, and the other 

 two situated, one on each side, near the compound eyes. 



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 mar- 

 gin 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 may be situated on a triangle indicated by 

 grooves or depressions or colorations, 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. Sometimes the term is applied to a triangle 

 indicated by color or depression in the dichoptic front. 



Ptilinum. In the Cyclorrhapha an inflatable organ ca- 



