XVI ON THE TERMINOLOGY OF DIPTERA. 



from the front to the back, and are called the fourth, fifth, and sixth 

 longitudinal veins. The hindmost vein of the anterior main trunk 

 and the foremost vein of the posterior main trunk, i. e. the third 

 and fourth longitudinal veins, are connected by a transverse vein 

 situated about the middle of the wing and called the small or mid- 

 dle transverse vein {vena transversa minor s. media). Using this 

 transverse vein as a starting-point, we cannot easily have any doubt 

 about the position of each of these six main longitudinal veins. 



The remaining neuration takes place in the following way : The 

 first of the four trunks emits a usually rather stout vein, forming 

 the anterior border of the wing ; it either runs round the whole 

 border of the wing, attenuating a little towards its end, and is called 

 the marginal vein (vena marginalis), or it only reaches as far as 

 the fourth or third, sometimes even the second or first longitudinal 

 veins, and is then generally called the costal vein (vena costalis s. 

 costa) ; both these expressions can be used as identical without any 

 fear of incorrectness. The costal vein is sometimes interrupted in 

 one or more places, thus forming a number of successive portions, 

 a structure most characteristic in several families. Besides this 

 vein, a second one proceeds from the foremost of the four trunks, 

 which, from its being frequently absent, is not counted with the 

 other longitudinal veins, but is called the auxiliary vein (vena aux- 

 iliaris) ; not far from its base, it is connected by the transverse 

 shoulder vein (vena transversa humeralis) with the costal vein. A 

 total or partial absence of the auxiliary vein, its structure, and the 

 peculiarities of situation which it has relatively to the first longi- 

 tudinal vein, are very characteristic marks for the distinction of 

 families and genera. 



The first and second longitudinal veins are usually simple, the 

 third being frequently forked; this furcation arises from its emitting 

 beyond the small transverse vein an anterior branch, which gene- 

 rally runs to the border of the wing between the second and third 

 longitudinal veins, and is therefore called the anterior branch of 

 the third longitudinal vein. Both branches together form the fork 

 of the third longitudinal vein, and that part of this vein which lies 

 between the small transverse vein and the point of forking of both 

 branches is called the handle (pednnculus) of this fork. Some- 

 times the foremost branch of the third longitudinal vein is con- 

 nected with the second longitudinal vein by a transverse vein, or 

 it rans into the second longitudinal vein instead of running to the 



