246 THE WINGS OF PLECOPTERA 



This is especially true as to the number of cross-veins and the branching of 

 the veins in the distal parts of the wings. On the other hand, the characters 

 presented by the trunks of the principal veins are quite constant. 



There is one characteristic of the wings of the Plecoptera that is so 

 constant that it may be considered an ordinal character. This is the fact 

 that in the wings of the adult the radial sector of the hind wings is attached 

 to media instead of to radius (Fig. 245). 



This switching of the radial sector of the hind wings is true only of the 

 venation of the adult. In the wings of nymphs the trachea Rg is a branch 

 of trachea R (Fig. 244).* 



There are certain features of the wings of Plecoptera, which although 

 not always constant, occur in so large a proportion of the members of the 

 order that they may be considered characteristic; these are the following, 

 all of which are represented in Figure 245 : 



The presence of the radial cross-vein (Fig. 245, r). The absence of 

 cross-veins in cell R and in the basal part of area Ri. Cross-veins are found 

 in cell R in Pteronarcys. 



The strengthening in the fore wing of the area between media and vein 

 Cui and of that between veins Cui and Cu2 by the development of many 

 cross- veins. 



The reduction of media to a two-branched condition. 



The reduction of the radial sector to a two-branched condition. This 

 fact is apparent only after an extended study of the wings of stone-flies. 

 In many cases, of which the form represented by Figure 245 is one, acces- 

 sory veins have been developed on vein R2+3, which appear to be the 

 primitive branches of the radial sector. But these accessory veins are 

 very inconstant in number and position. I am convinced that only the 

 first forking of the radial sector, the division of it into veins R2+3 and 

 R4+5 has been retained from the stem form of the order. The farther 

 branching of either of these veins is too erratic to be considered primitive, 

 frequently differing in the wings of opposite sides of an individual insect. 



The unbranched condition of the first anal vein in both fore and hind 

 wings; rarely, as in some species of Pteronarcys, the first anal vein is 

 branched at the tip. 



In the unbranched condition of the first anal vein the Plecoptera 

 resemble the Orthoptera. But the Plecoptera differ from the Orthoptera 

 in that in the expansion of the venation of the anal area only accessory veins 

 are developed; intercalary veins are not found in the Plecoptera; although 

 in the Australian genus Eusthenia (Fig. 246) there are what appear to be 



*In spite of this fact, Klapalek ('12) states: "Im Hinterfliigcl finden wir in dem 

 reifen Fliigel kcinen Sektor radii, dafiir ist die Media zweimal gegabelt." In carrying 

 out this idea he incorrectly labels the branches of the radial sector and of media in his 

 figure of the wing- venation. This is a remarkable example of a lack of appreciation of 

 the value of ontogenetic facts. 



