|no.i33i. dragox-fly wing venation— needham. 755 



f (fig. 42) shows how such a brace is evolved out of the boundaries of 

 [Ordinary cells, 



I This brace joins the main vein with the elbow in the branch, thus 



^correcting the mechanical weakness of the unilateral fork. Aside 



ifrdin the bridge, such a brace is developed in Rhlnocyjyha {^g. 33), 



extending the attachment of vein Rs along vein J/1+2 toward the 



arculus;" also, in Chalcopteryx and many Calopteryginse; again, at the 



I proximal end of vein 31^ in Neurobasis (fig. 43) and a few closely 



related genera,^ as it was, also, in some fossil species referred to 



\ Stenoj)hlebia; again, at the proximal end of vein M^^ in Nasi?esch7ia 



\ pentacantJia (Plate XXXIX, fig. 1) and many other Anisoptera, in 



which, however, it is, as a rule, imperfectly developed or not developed 



at all; and again in Philoganga montana (fig. 44). The bridge itself 



seems to be still extending proximally in some LestinaB (Plate LI, 



figs. 6 and 7, and LIII, fig. 1), its slender proximal end being often 



unattached to vein M^ + 3. 



ji Here we have strong side light upon the early history of that most 

 distinctive peculiarity of dragonliy wings, the crossing of the radial 



Fig. 44.— Wing of Philoganga montana Selys. 



sector over two branches of the media and the development of the 

 bridge. Why this crossing should ever have taken place is left 

 entirel}^ unexplained. That it was established very early in the his- 

 tory of the group is indicated by its occurrence in very early nymphal 

 life, crossing being no longer possible after the internal fusion of the 

 hypoderm cells to form the tracheal channels. But, once across, it 

 formed a weak unilateral fork upon the posterior side of vein J/3, 

 while occupying a field where strong developments were possible. 

 Hence a strong bridge was evolved for its support, and the develop- 

 ment of the bridge doubtless followed the lines we have just indicated. 

 The proximal end of the bridge in most living species joins vein M^..^ 

 directly, but in most Gomphinfe it appears as if forked, and in the 

 fossil Heterophlebina? it joined vein J/, directly. 



In the singularly isolated fossil genus iSfenoph/ehJa, whose venation 

 has been figured in a very detailed manner by Hagen,*" there is a con- 



«It will be remembered that the trachea Rs here springs directly from trachea 

 3/1 + ^. The oblique vein (r/, fig. 34) marks the place of its origin; on the proximal 

 side is the added brace {hr^). 



6 It will be observed that this brace is better developed in the fore wing than in 

 the hind in Neurohasi}<; similarly, the other brace just mentioned for Choh-opteryx; 

 two further exanijiles, therefore, of unequal development of fore and hind wings. 



'•' Pala?ontographica, XV, pi. iii, tig. 1. 



Proc. N. M. vol. xxvi— 02 51 



