THE WINGS OF NEUROPTERA 163 



insects of the genus Acanihaclisis (Fig. 156, rca). This area may be 

 termed the radial cuneate area. The size of this area and the arrangement 

 of the veins within it afford characters of considerable taxonomic import- 

 ance, which win doubtless be used in the future. 



In many cases, especially in the Myrmeleonidas, but less frequently in 

 the Ascalaphidse, this area is bounded by two forks of the first branch of the 

 radial sector (Fig. 156) ; in others it is irregular in form and appears to be 

 behind the terminal portion of this branch of the radial sector (Fig. 157). 



In many of the Myrmeleonidas the only indication of the expansion of 

 this area is a splitting back of the tip of vein R5 to a somewhat greater 

 distance than are split the tips of other veins in this region of the wing 

 (Fig. 158). 



All intergrades between a slight splitting of the tip of vein R5 and a 

 broadly expanded radial cuneate area exist in the Mjameleonidae. When 

 the splitting of vein R5 has progressed to any considerable extent, accessory 

 veins have been developed on one or on both branches of vein R5 (Fig. 156). 



Some students of the Myrmeleonidae in our laboratory have suggested 

 that when the first branch of the radial sector is forked the two divisions of 

 this branch represent R4 and R5 respectively, thus indicating that the 

 suppression of the dichotomy of the radial sector is by a coalescence of 

 these two veins, which in these cases is not quite complete. 



It is quite important to determine if possible, which of these two views 

 is correct ; as the terminology of the branches of the radial sector depends 

 on the view adopted. If the suppression of the dichotomy is by the switch- 

 ing of the base of vein R4 from vein R5 to vein R2+3, the first branch of the 

 sector is vein R5 ; the second, R4 ; the third, R3 ; and the following branches 

 are accessory veins. If the suppression of the dichotomy is by the coales- 

 cence of veins R4 and R5, the first branch of the sector is vein R4+5; the 

 second, R3; and the series of accessory veins begins with the third branch. 



The most conclusive evidence that the former view is the connect one is 

 given in the earlier section of this chapter refen^ed to above ; but there are 

 other considerations that support this view. 



All of the data so far observed on the suppression of the dichotomy of 

 the radial sector indicate that the forces that make for this suppression act 

 to spread apart veins R4 and R5 so that they become separate and parallel 

 branches of the sector. This has been shown in the case of the Sialidae and 

 of the Hemerobiidffi. In the Osmylidaj we have seen that the switching of 

 vein R4 to vein R2+3 is merely a modification of this process; veins R4 and 

 R5 split apart for a short distance and then the base of vein R4 takes a short 

 cut to vein R2+3 via a cross-vein. The suppression of the dichotomy by the 

 coalescence of veins R4 and R5 would be the result of a force acting in the 

 opposite direction, one that tends to bring veins R4 and Ro together instead 

 of splitting them apart. That such a force has acted seems improbable 



