ON THE TERMINOLOGY OF DIPTERA. XV 11 



border of the wing, and thus has the appearance of a transverse 

 vein. 



In most Diptera there is no other connection between the third 

 and fourth longitudinal veins except the small transverse vein, and 

 we can cross the wing between the third and fourth longitudinal 

 veins in its whole length without meeting another vein but the 

 small transverse vein. But in some families the fourth longitudinal 

 vein, abandoning towards its end its former direction, turns to the 

 third longitudinal vein and reaches it either at its end or a little 

 before it, constituting thus a second connection : there is a rarer 

 case when that connection is effected by a transverse vein placed 

 distinctly on the fourth longitudinal vein. A third connection 

 between the fourth longitudinal vein and the anterior main trunk 

 is formed in some families in the neighborhood of the base of the 

 wing; often there is only a transverse fold running obliquely from 

 the fourth longitudinal vein to the first; in some families it incras- 

 sates into a transverse vein. 



The three longitudinal veins belonging to the second main trunk 

 usually begin to diverge quite near the base of the wing; the hind- 

 most of them, i. e. the sixth longitudinal vein, is often distinctly 

 seen as the continuation of the common trunk, while the two ante- 

 rior ones uniting with their bases seem to form a kind of loop 

 which touches the main trunk only at one point. As frequently, 

 the fifth longitudinal vein represents a distinct continuation of the 

 main trunk; in some families all the three longitudinal veins appear 

 in equal distinctness as its branches. Between the fourth and the 

 fifth longitudinal veins there are in general two transverse veins, 

 which divide the space of the wing, included by the fourth and fifth 

 longitudinal veins, into three parts. The first of these transverse 

 veins is the anterior basal transverse vein (vena transversa basalts 

 anterior s. venula basalis anterior), the absence of which is charac- 

 teristic for some families ; the second is usually the longest trans- 

 verse vein of the wing and is of the highest systematic value ; it 

 is called the posterior transverse vein (vena transversa posterior s. 

 venula posterior). Not unfrequently another vein starts from its 

 middle, running to the border of the wing; it cannot be considered 

 as a longitudinal vein, and is called the anterior intercalary vein 

 (vena intercalaris anterior). It must not be confounded with a 

 branch emitted in some Diptera from the posterior side of the 

 fourth longitudinal vein before its tip. 



