96 



THE PALEONTOLOGICAL DATA 



Two types of the radial sector are found in the older insect remains, and 

 in living insects as well. These may be defined as the pectinate radial 

 sector and the dichotomously branched radial sector, respectively. 



In the pectinate radial sector the principal branches form a single series 

 of approximately parallel veins, resembling in their arrangement the teeth 

 of a comb (Fig. 86, a). In the dichotomously branched radial sector, the 

 principal branches, typically four in number, show a dichotomous arrange- 

 ment; that is the sector divides into two stems and each of these in turn 

 divides into two branches (Fig. 86, h). 



A good example of the pectinate type of radial sector is that of Steno- 

 dictya (Fig. 77). The genus Stenodictya is placed by Handlirsch in what 

 he considers the most generalized family of the Palaeodictyoptera, the 

 Dictyoneuridag. In another genus, Polioptenus, both types occur; and in 

 all other known genera of this family of which the wings are siifficiently 



well-preserved to show the 

 Sl- form of the radial sector 



the principal branches show 

 a dichotomous arrangement. 

 There are ten of these 

 genera; eight of which are 

 from the Middle Upper Car- 

 boniferous ; while all of the 

 species of Stenodictya are 

 from the Upper Upper Car- 

 boniferous. 

 In other words, all of the older Dictyoneuridae, except Polioptenus in 

 which both types of radial sector are found, had the radial sector dichoto- 

 mously branched. 



It seems evident, therefore, that in the later appearing Stenodictya from 

 the Upper Upper Carboniferous, the radial sector has not retained its 

 primitive form; and that the pectinate type was derived from the dicho- 

 tomously branched type. In the chapter on the wings of the Neuroptera 

 the suppression of the dichotomy of the radial sector is discussed. 



A careful tabulation of the data exhibited by the Palasodictyoptera as a 

 whole shows that the type of branching of the radial sector can be deter- 

 mined in representatives of twenty of the twenty-two families included in 

 the order ; and that in fourteen of these twenty families only the dichoto- 

 mous type is found; in two, only the pectinate type; and in four both 

 types existed. 



Let us now study the evidence presented by the oldest insect remains 

 that are known, those from the Lower Upper Carboniferous. Of these 

 there are eight, six from the United States, and one each from Wales and 

 from Germany. 



Cui' Mi, 



Fig. 87. — Metropator pussillus (After Handlirsch). 



