THE WINGS OF INSECTS 



411 



has been lost in this genus, but is well-preserved in Macroxyela and is, there- 

 fore, represented in the figure. 



While the tips of the branches of the radial sector have migrated away 

 from the apex of the wing, the bases of these branches coalesce in the oppo- 

 site direction ; from these two causes results the transverse bracing of the 

 radial area of the wing, which is a very characteristic feature of the vena- 

 tion of the wings in this order. 



The details of these changes will be made clear by an examination of 

 Figures 427a and 427b. 

 The former represents the ~Z!I^^^*^~~~-^ 



primitive mode of branch- p^m^m^^ — ■" "x'L^ ' ^\/?! 



ing of the radius; the 

 latter, the radial area of the 

 typical hymenopterous wing 

 (Fig. 425). In the hymen- 

 opterous type veins R2+3 

 and R4+5 of the primitive 

 type coalesce so far that 

 the branches of the sector 

 arise from a common stem; 

 and the tips of all of them 

 have mo\'ed away from the 

 apex of the wing, veins Ro 

 and R3 following the costal margin of the wing; and veins R4 and Rs fol- 

 lowing vein Ml. 



In the Hymenoptera the radial cross-vein is frequently preserved ; it is 

 marked r in figure 425. 



In descriptions of wings of Diptera and of Lepidoptcra, in those cases 

 where compound veins are fomied by the coalescence of adjacent \-eins, 

 the compound vein is designated by a term that indicates its composition. 

 Thus in Rhyphits (Fig. 410), the vein formed by the coalescence of veins R2 

 and R3 is designated as vein R2+.3; and in the wing of Eiilonchus (see your 

 drawing of this wing) the tmited tips of veins Cui and Mg is designated as 

 vein Cui + M3. 



When it is desired to indicate the composition of a compound xexn it 

 can be readily done by the application of this system. But in descriptions 

 of hymenopterous wings, where a compound vein may be formed by the 

 coalescence of several veins the logical carrying out of this plan would result 

 in a very cumbersome terminology, one that it is impracticable to use in 

 ordinary descriptive work. In such cases the com])ound vein is designated 

 by the term indicating its most obvious element. Thus, for example, in 

 the fore wing of Pamphilius, where veins M4, Cui and Cuo coalesce with 

 the first anal vein, the united tips of these veins is designated as vein ist A, 



Fig. 427.- 



-Diagrams of the radius: 

 b, hymenopterous. 



a, typical; 



