592 THE PANORPOID COMPLEX, iii., 



Section vii. Veinlets and Cross-Veins. 



The distinction between a veinlet and a cross-vein has already 

 been made in Part 2 of this work (25, p. 628) . A veinlet is a 

 branchlet of a main vein; it- is preceded by a trachea in the 

 pupal wing, and carries macrotrichia in all archaic groups. A 

 cross-vein, on the other hand, is a transverse vein which is de- 

 veloped independently of the branching system of any main 

 vein; it is not p receded by a trachea in the pupal wing (except 

 in a few cases of high specialisation), nor does it carry macro- 

 trichia in any archaic group. 



Veinlets originally come off obliquely from their main veins. 

 But they frequently tend to assume a transverse position, and so 

 come to take on the characters of cross-veins. In the course 

 of reduction, they may lose their precedent trachea?, or their 

 macrotrichia, or both. In such cases, their true nature is only 

 to be determined by a study of their homologues in more archaic 

 types . 



• Cross-veins originally formed transverse struts or supports 

 between main veins or their branches. They may, however, 

 become specialised so as to take up an oblique position, and 

 may even become lengthened, so as to appear like veinlets or 

 even branch-veins. In a few highly specialised cases, they may 

 gain an underlying trachea in the late pupal wing, or at meta- 

 morphosis. In two groups, viz. in the higher Planipennia and 

 the Raphidioidea, they gain macrotrichia. In such cases as 

 these, their true nature can only be determined by a study of 

 their homologues in more archaic types. 



In certain cases, as, for example, when two main veins fuse 

 together for pai't of their courses, a small portion of a main vein 

 may become bent at an angle to the rest, and so take on the 

 appearance of a veinlet. Further specialisation may cause the 

 trachea underlying this vein to weaken, so that it becomes a 

 veinlet to all intents and purposes. It may even lose its trachea 

 and macrotrichia, and take on a transverse position, so as to be 

 indistinguishable from a true cross-vein, except by a study 

 of its origin in more archaic forms. 



Main veins which have undergone these specialisations are the 

 following : — - 



The basal pieces of M 5 and Cu 1( during the evolution of the 

 cubito-median Y-vein. {See Section iii.) . 



The basal piece of M 4 in Lepidoptera (Section vi.), or of 



