32 DIPTERA OF NORTH AMERICA. [PART IV. 



cross-vein, then along the discal cell, between the two intercalary 

 veins, to the posterior margin (see 1. c. fig. 3, the wing of Empis, 

 which in this respect resembles that of the Tipulidse, and com- 

 pare it to the wing of Ortalis and to the diagram which I give 

 below). Thus, if we force upon the Tix>ulidse the terminology 

 introduced originally for the families of Diptera with a less de- 

 veloped venation, we meet with inextricable difficulties. But 

 there is no more reason for doing this than for following the 

 opposite course, adopting a terminology for the Tipulidae first 

 and forcing it afterwards upon the Muscidse. It is perfectly 

 arbitraiy at which end of the system of Diptera we begin to 

 trace out the homologies of the venation. This study of the 

 homologies has two distinct aims* in view : the scientific aim of 

 showing that the ground-plan of the venation is the same in all 

 the families of the order ; and the practical aim of adopting a 

 terminology for descriptive purposes. We cannot carry out a 

 terminology on solely theoretical grounds ; we will have to vary 

 the details of it according to the peculiarities of structure occur- 

 ring in different families, the main plan remaining the same. This 

 is done in all the departments of zoology, and I do not see why 

 the venation of the Diptera should be treated differently. 



In accordance with these views, I call fifth longitudinal vein 

 the whole vein immediately following the second basal cell and 

 the last of the posterior cells ; I call great cross-vein (in contra- 

 distinction from the posterior cross-vein of the Muscidse) the 

 cross-vein connecting the fifth vein with the vein preceding it. 

 T\\Q fourth vein, I look upon as including the discal cell between 

 its two main branches.* The posterior of these branches is almost 

 always forked (the posterior branch of this fork corresponds to 

 Mr. Loew's 2^osterior intercalary vein, v, in the wing of Empis, 

 Ilonogr. I, p. xxiv, fig. 3) ; and the cross-vein, connecting this 

 fork with the anterior branch, closes the discal cell ; hence, when 

 the discal cell is open, through the disappearance of this cross- 

 vein it coalesces with the second posterior cell (as in Tab. I, fig. 1), 

 or with the third, when there are five posterior cells (as in Tab. 

 II, fig. IT). Such is the case with the majority of the genera 

 which have the discal cell open, as Orimarga, EmjJeda, Crxjpto- 

 labis, Ej'ioptera (subgenera : EriojJtera and Molopjhilus), Plec- 



' In tins I follow Dr. Suliiuer's views. 



