24 DIPTERA OF NORTH AMERICA. [PART IV. 



cross -vein near the tip of the auxiliary vein, posterior to the origin of 

 the second vein. Eyes glabrous. The normal number of the antennal 

 joints is six in the male and not more than ten in the female. Tibia; with 

 spurs at the tip ; empodia distinct ; ungues generally smooth. 



VI. Amalopina. Two submarginal cells ; discal cell closed or open ; 

 subcostal cross-vein far removed from the tip of the auxiliary vein, an- 

 terior to the origin of the second longitudinal vein. Tibiae with spurs at 

 the tip ; empodia distinct. Eyes pubescent ; front usually with a more or 

 less distinct gibbosity. Normal number of antennal joints sixteen (seldom 

 17), or thirteen. 



The second of these groups is called artificial, because it is 

 destined to receive all the genera with a single submarginal 

 cell which, at the same time, have sixteen-jointed antenna?. All 

 such genera are so very peculiar in their characters, that it is 

 natural enough to isolate them from the first section ; but with 

 all that, most of these genera do not show any relationship to 

 each other and their juxtaposition is therefore artificial. The 

 connecting links between them may not have been yet discovered, 

 or they may have been lost in the course of geological ages ; 

 nevertheless, the adoption of this artificial group will be found of 

 great advantage in the system. If it should be proved that one 

 of these genera is related to some genus of another section, it 

 will have to be removed to that section. Thus, in the genus 

 Cladolipes Loew, closely related to Anisomera, one of the 

 branches of the second vein has disappeared, and hence the genus 

 has only a single submarginal cell. Nevertheless, as the natural 

 relationship of this genus is evident, we place it among the 

 genera with two submarginal cells. The aim of all classification 

 is to increase our knowledge of the structure of organic beings 

 by illustrating their natural relationship. If the natural relation- 

 ship of some organic form be obscure, we may, for the sake of 

 convenience, locate it provisionally on account of some artificial 

 character ; but this provisional state has to cease, as soon as the 

 true relationship is found out. In this sense, the location of 

 several of the genera of the second group may be only provisional 

 and connecting links between them and the other sections may 

 yet be discovered. 



The other sections, as far as known, have very well marked 



