256 E. H. Sellards — Types of Permian Insects. 



many modern genera. Lines of matched cross veins are seen 

 also in the basal part of the wing (fig. 2). 



JYodus and stigma. — On the Paleozoic genera thus far 

 known, both nodus and stigma are apparently lacking. Since 

 the nodus is recognized as merely a strongly developed cross 

 vein running from the costa to the radius, at which cross vein 

 the subcosta usually terminates, the existence of a weak nodus 

 on some of the more specialized late Paleozoic genera need 

 occasion no surprise. It is to be expected that the subcosta 

 will in early types reach beyond the cross vein, the nodus 

 appearing simply as a strong cross vein from costa to sub- 

 costa, matched with a similar strong cross vein from subcosta 

 to radius. Much the same may be said of the stigma, a strength- 

 ening structure thus far not observed to occur on these early 

 Odonates. 



The ar cuius. — A very conspicuous structural feature of the 

 modern dragon-fly wing is the arculus. As is well known, the 

 arculus of this, as of other orders in which it occurs, consists 

 in part of the media directed obliquely at its point of origin 

 from the radius, and in part of a strong cross vein from the 

 media to the cubitus. The arculus, as a conspicuous feature, 

 is absent from the wings of Paleozoic dragon-flies ; yet the ele- 

 ments which compose it are there and in their respective posi- 

 tions. For the formation of a characteristic arculus there is 

 needed scarcely more than a slightly accentuated bend of the 

 media at its separation from the radius and a correlated 

 strengthening of the cross veins bearing the chief stress in 

 the support of the media. Along with this will go naturally 

 the more or less complete disappearance of the now unneces- 

 sary accessory cross veins. 



Triangle and (quadrangle. — The structures known as trian- 

 gle in the Anisoptera and as quadrangle in Zygoptera result, 

 as in the case of the arculus, from specialized cross veins in 

 conjunction often with a modified course of the adjoining main 

 veins. The base of the triangle of the Anisoptera is formed 

 by the cubitus, and results from an abrupt bend of that vein 

 just beyond the arculus. The sides of the triangle are formed 

 each by a cross vein running from the cubitus to the media. 

 With the Zygoptera, in which the bend of the cubitus is less 

 abrupt, there is naturally, a less well-marked structure in this 

 region, and as the arrangement of cross veins gives to this area 

 a quadrangular rather than a triangular shape, Needham has 

 proposed to designate it as the quadrangle. As has been noted 

 above, the cubitus of the known Paleozoic forms agrees with 

 that of the generalized modern Odonates in its sinuous, rather 

 than abruptly bent course. With the most generalized of 

 modern Odonates the quadrangle or triangle, as conspicuous 



