THE WINGS OF NEUROPTERA 147 



The marginal accessory veins in many of the Neuroptera differ remark- 

 ably from those of the other orders of insects in which this type of veins is 

 found in the fact that they form a regular border of qtiite uniform width. 

 This is illustrated by the veins of the outer margin of the wings of Osmylus 

 hyalinatus (Fig. 137). 



In some cases there is a splitting of one or both forks of a vein that has 

 been split, thus forming a second rank of marginal accessory veins; this is 

 the case in the wings of Polystoechotes, figured later (Fig. 141). 



The distinctive characteristic of marginal accessory veins is their 

 instability; they vary in number and in length in different individuals of 

 the same species, and even in the wings of the two sides of the same individ- 

 ual. But when the splitting of a vein has progressed siifficiently far it 

 becomes fixed, and what at first, phylogenetically, was merely a marginal 

 accessory vein become transfonned into a definitive accessory vein. 



The beautiful symmetry of the borders of marginal accessory veins in 

 certain members of this order is due not merely to the nearly equal length of 

 these veins but also to the fact that the two forks of each forked vein 

 occupy bilaterall}^ s^mimetrical positions. It follows that neither fork can 

 be regarded as accessory to the other, they are sister veins, both accessory 

 to the stem from which they were derived. 



The suppression of the dichotomy of the radial sector. — Correlated with 

 the extensive dvelopment of accessory veins in the Neuroptera, there has 

 resulted in nearly all of the families of this order the production of a pectin- 

 ately branched radial sector; that is, this vein is so modified that it consists 

 of a supporting stem upon which are borne a greater or less number of 

 parallel branches. This is shown in most of the figures of wings illustrating 

 this chapter. This is a distinctive characteristic of this order; in no one 

 of the other orders of living insects in which accessory veins occur is a well- 

 developed pectinately branched radial sector found. Such a sector existed, 

 however, in many of the Palajodictyoptera. 



The typical radial sector is dichotomously branched, being divided into 

 two chief branches, veins R2+3, and R4+,'j, and each of these in turn is 

 divided into two branches, the former into veins Ro and R3 and the latter 

 into veins R4 and R5. 



The transformation of a dichotomously branched radial sector into one 

 that is pectinately branched was discussed by Comstock and Needham, 

 and the process by which this transformation is attained was termed by us 

 the suppression of the dichotomy of the radial sector. In the course of our 

 discussion, we pointed out that there are three ways in which this result 

 may have been attained: first, by the splitting apart of veins R4 and R5 

 so that they arise separately from the supporting stem of the pectinate vein 

 thus formed; second, by the switching of the base of vein R4 to vein R2+3, 

 following a cross-vein, this also resulting in the two veins arising separately 



