THE WINGS OF PLECOPTERA 



253 



branched. But from the facts given above, this condition of the radial 

 sector and of media may be considered as typical of the primitive plecop- 

 terous wings. 



In Nemoiira Comstock and Needham failed to find a costal trachea; 

 but as we found a well-developed costal trachea in the closely allied genus 



Fig. 252. — Wings of a nymph of Nemoura (From C. &■ N.). 



Tceniopteryx, we may assume that this trachea was present in the primitive 

 type of the order. 



The more important features of the primitive type of tracheation of the 

 wings of the Plecoptera may be described as follows: 



The absence of tracheee in the cross-veins. 



The absence of a basal transverse trachea, and correlated with this, the 

 origin of the medial trachea from the costo-radial group of tracheae. 



A typical two-branched subcostal trachea without accessory tracheae. 

 The trachea Sci extends to the margin of the wing; the trachea Sc2 extends 

 towards trachea Ri until it nearly reaches it, and then curving away from 

 this trachea extends to the margin of the wing. 



A typical radial trachea except that its sector is only two-branched. It 

 should be noted that the radial sector trachea of the hind wing, like that of 

 the fore wing, springs from the main stem of the radial trachea. But the 

 switching of the radial sector of the hind wing to media is foreshadowed by 

 the course of the forming vein (Fig. 252). 



A typical medial trachea except that it is only two-1) ranched. 



