INTRODUCTION. XI 



brilliantly and beautifully colored with green and piirple 

 markings. Sometimes the enlargement of the facets is on the 

 anterior portion and common to both sexes, as in the Asilidae. 

 The larger number of flies have the eyes bare, or pubescent 

 only when seen under high magnification. Very often, how- 

 ever, the whole or part of the eyes is covered with erect pile 

 or hair, which always finds its greatest development in the 

 male sex. The pilosity may be sparse or dense, short or long. 



Ocelli. On the up])er part of the head, between the com- 

 pound eyes there are three simple, small eyes, present in most 

 diptera, and called the ocelli. They 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 whose apex is in front ; sometimes tliey are 

 located in a nearly straight line transversely, or, the middle 

 one may be absent, and the other two situated one on each 

 side close to the compound eyes. 



Antennw. Ko other organs furnish so many or so important 

 characters in the classification of the diptera as do the anten- 

 nae or feelers, as they have been sometimes called. The num- 

 ber, shape, and arrangement of the joints offer, not only 

 s])ecific and generic cliaracters, but in some cases family char- 

 acters as well. Only in exceptional cases is the number less 

 than three, and there may be as many as thirty-six, it is said. 

 Through all the Cyclorrhapha the number three is constant, 

 with tlie exception of the Phoridffi, and the Pupi])ara. In the 

 Nematocerous Orthorrhapha the number is usually from eight 

 to sixteen, the first two of which form the scape, and which 

 are always more or less differentiated from the remainder, 

 which constitutes t\\Q ^ficKjellum. Osten Sacken has pro})Osed 

 to call those flies which have the antennae long and frequently 

 bearing whorls of hairs, especially in the males, the true 

 Xematocera, in distinction from the anovialous Nematocera, 

 in which the antennse are shorter, destitute of whorls of hairs 

 and with the joints pressed close togctlier. Upon the antenna! 



