750 PROCEEDINGS OF THE NATIONAL MUSEUM. 



dition of the crossing of the radial sector so curious one hesitates at 

 attempting- to explain it on the ground of published figures that were 

 draAvn without ontogony in mind. It looks as if, a little bej^ond the 

 iKxhis, the radial sector had crossed over but one branch of the media 

 and had then developed a short supporting bridge; as if a stage a little 

 less primitive than that shown for trachea in fig, 1, B^ had been the 

 permanent condition in this genus. But perhaps the interpretation of) 

 homologies given by Hagen is correct (naturally it is the only one thati 

 would occur to him), in which case the bridge is developed as a sup- 

 port to vein J/j. and the radial sector is shifted proxinially at its; 

 base — a very curious state of things for Anisoptera! And Stenopldebid 

 is distinctly anisopterous, notwithstanding its lack of differentiation 

 between fore and hind wing. Aside from the characters just dis- 

 cussed, it is a very curious form in its sickle-shaped wing tips, its 

 dislocated nodus with the subcosta descending upon the radius,- 

 its undifferentiated subtriangles, its triangles transversely placed im 

 both fore and hind wings, and its remarkably developed trigonal 

 supplements. 



If I mistake not, it is to the readjustment of stress after the crossing 

 of the radial sector that is due the curious bendings of veins J/g and 

 J/^ in many fossils (notalily in Cymatophlehia)^ in some living forms 

 of ancient aspect {^l^hyUopetalla^ fig. 26, and Cldorogoriq^hus^ fig, 24, 

 etc.), and in some of the more generalized members of groups at present 

 dominant ( Gomphiexehna. Plate XXXVII, fig. 1 ; I)!(lymops, Plate XLI,i 

 fig. 2, etc.). This bending is very different from the undulation of the 

 radial sector in certain Libellulinse. This is primitive, defective, and 

 early disappears in the dominant groups, or becomes transformed into 

 something ver}' ditt'erent and more useful. That other is recent, local, I 

 and is but one among several factors in the bracing of the wing area 

 in which it occurs. 



A\'hen vein Cu became unilateral on its posterior side, the weakness 

 of this fork was corrected })y the apposition of the distal end of vein 



It must be borne in mind that dynamic control in vein development, 

 dynamic genesis, or whatever we call it, may be but the result of the 

 natural elimination or subordination of those variations w^hich do not' 

 tend toward the mechanical perfecting of the machinery of flight. 



VI. SUMMARY AND CONCLUSIONS. 



1. In the course of this study I have come upon numerous mani- 

 festations of developmental dynamics: 



{a) I have shown the operation in dragon-fly wings of far-reaching 



"This type of I)racinff, which is so confusing of homologies as generally to require 

 ontogenetic study for their unraveling, is of very cominon occurrence near the apex 

 of grasshopper wings. 



