70 



THE GENERAL FEATURES OF WINGS 



Fig. 56. — Wings of a braconid. 



into the composition of a serial vein, and where, owing to extensive modifica- 

 tions of the wing-venation, it is impossible to determine completely the 

 composition of the resulting vein. 



The problem of devising a method of designating serial veins now arises. 

 In cases where the vein consists of only two elements it is a simple matter. 

 The serial vein in the braconid wing described above, consisting of the 

 medial cross-vein (m) and vein M2, can be designated as m & Mo. The 



sign & is used instead of 

 + , as the latter is used 

 to indicate compound 

 veins f onned by the coal- 

 escence of veins side by 

 side, as indicated on an 

 earlier page. 



In those cases where 

 sections of several veins 

 enter into the composi- 

 tion of a serial vein, it is 

 usually possible to deter- 

 mine the basal element 

 and also that forming 

 the tip of the serial vein, even though it is impossible to determine defin- 

 itely the veins that are represented by the intermediate portion of the 

 vein. In such cases the serial vein can be designated by the abbre\'iation 

 of the name of the basal element connected by a dash with the abbre- 

 viation of the name of the terminal portion. Thus a serial vein, the basal 

 element of which is the cubitus and the terminal element vein Mi is desig- 

 nated as vein Cu-Mi. A serial vein thus formed exists in the hind wings of 

 certain ichneiunon-flies (Fig. 54). This method of designating serial veins 

 is suggested by Professor Bradley as a substitute for that used in his 

 monograph of the Evaniida? (Bradley '08). 



The increase of the number of wing-vems. — In many insects the wings 

 are furnished with a larger number of veins than that which we believed to 

 have been characteristic of the wings of the primitive winged insects; the 

 evolutionary process by which this change has been brought about is 

 termed specialization by addition. 



This multiplication of veins is due either to an increase in the number of 

 the branches of the principal veins by the addition of secondary branches, 

 or to the development of secondary longitudinal veins between these 

 branches. In no case is there an increase in the number of principal veins. 

 In some cases the specialization by addition has taken place only in the 

 preanal area; in others it is confined to the anal area; in still others it has 

 occurred in both areas ; and in many insects a reduction in the number of 



