NO. 1331. DRAGON-FLY WING VENATION— NEEDHAM. 757 



mechanical principles, in accordance with which the potentially hex- 

 agonal areole.s of the wing are arranged in the spaces they must 

 occupy, as are other wholly unrelated potentially hexagonal struc- 



\ tures in other organisms. 



\ (h) I have shown that there is developed in this group, several 



\ times independently, a form of Aving that is elsewhere most etlicient — 



[ a wing broad at base and long and pointed at the apex, rigid at the 



, front and pliant toward the rear margin — a wing combining the prin- 



, ciple of the aeroplane with that of the scull. 



I (c) I have shown that the development of wing braces follows 



■ strictly mechanical principles, analogous braces being repeatedly 

 developed out of homologically different parts. Many examples have 



ji been cited within the order, and one without, in the comparison of 



[ Odonate and Ascalaphid wings. 



i 2. In this study I have indicated processes concerned with the 



i development of these wings which will probabl}^ be found affecting 



I the evolution of insect wings in general: 



j {(/) I have shown that there are two kinds of specialization in 



! operation throughout the order — vein shifting, concerned with secur- 

 ing- advantageous position of the parts, and vein differentiation, con- 

 cerned with the strengthening of the most important veins by an 

 economical use of all strength-giving wing material. The former 



1 alone seems to have been made use of in venational studies hitherto; 



I the latter is often a more sure criterion of the degree of specialization. 



I (h) I have offered a hypothetical explanation of the progressive 



I differentiation between veins and membrane. 



3. In the study of dragon-fly wing venation only the comparative 



I anatomy of the adult wings has been drawn upon hitherto. I have 

 added the ontogenetic method, beginning my study of the veins with 

 that of their antecedent trachea. I have found this method to fur- 

 nish most satisfactory evidence as to what was the primitive position 

 of the veins in almost every part of the wing for all the principal 

 groups of the Odonata. This, followed by careful study of adult 

 wings, both recent and fossil, has enabled me to make some slight con- 

 tributions to Odonatology proper: 



(a) I have for the first time homologized in detail the parts of the 

 dragonfly wing- with those of the wings of insects of other orders, 

 applying the simple Redtenbacher terminology, retaining the special 

 terms already in use for parts not represented in other orders, simpli- 

 fying some of them, and adding a few new terms for parts not hitherto 

 designated by name. 



(?>) Homologies within the order have been pretty well understood 

 for a long time, thanks to the labors of many able entomologists, 

 among whom may be mentioned Hagen, Walsh, and especially that 

 lifelong student of this order, Baron de Selys Longchamps. In 1893, 



