254 . THE WINGS OF PLECOPTERA 



A typical two-branched cubital trachea. 



Three unbranched anal trachece in the fore wing. 



Three anal tracheas in the hind wing; the first anal trachea is un- 

 branched; the second and third anal tracheae are each two-branched. 



The characteristic features of the venation of the preanal area of the 

 wings of the adult Nemoura (Fig. 248) need not be described in detail, as 

 the veins follow quite closely the courses of the tracheas that precede them, 

 which have been described above. There is an anastomosis of veins Sco 

 and Ri in the region where the trachea Sco was closely parallel with trachea 

 Ri; and the radial sector of the hind wing has been switched to media. 



There are certain features of the venation of the anal area of the fore 

 wing that merit special attention. In Nemotira, and in fact in the greater 

 number of the genera of this order, the anal area of the fore wings contains 

 three, and only three, unbranched anal veins. The first and second anal 

 veins are connected by a cross-vein, near the base of the wing, which closes 

 the basal anal cell. The third anal vein either coalesces with the second at 

 the base of the wing; or, when the two veins are separate at the base, they 

 anastomose opposite the basal anal cell.* 



(/) THE METHODS OF SPECIALIZATION OF THE WHSTOS OF THE PLECOPTERA 



If we accept the conclusion that the type of wings represented by 

 Nemoura can be taken as illustrating the primitive plecopterous wings, 

 there follows the conclusion that within this order the specialization of the 

 wings has proceeded in opposite directions in different members of the order. 

 In some there has been a farther reduction of the wing-venation; in others, 

 a specialization by addition. 



An equally remarkable fact is that differences in the direction of the 

 specialization of the wing-venation do not indicate important divisions of 

 this order. Within a single family forms exist in which there has been a 

 reduction of the wing-venation and also others in which the specialization 

 has been by addition. 



A few illustrations of the more striking modifications of the primitive 

 plecopterous type follow. 



Specialization by reduction. —There are many cases of specialization by 

 reduction in this order; a common example is the reduction of the radial 

 sector of one or of both pairs of wings to an unbranched condition; this 

 occurs in each of the five families. 



*Much use is made by systematists of the characters presented by this part of the 

 anal area. When the coalescence or anastomosis of the second and third anal vein 

 docs not extend beyond the end of the cell, as in Iso^eniis (Fig. 245), it is said that 

 "two simple veins extend from the cell below," when the union of these two veins 

 extends beyond the limits of the cell, as in Nemoura (Pig. 248), it is said that "a single 

 forked vein extends from the cell below." The inclusion of the word below in these 

 expressions is to indicate that the first anal vein is not included in the enumeration. 



