435 



at the nodus, crosses the first median vein, the first vein caudad of it, at 

 the point where the second median vein separates from the first, fol- 

 lows the second vein for a short but indefinite distance, being fused 

 M'ith it, and then crosses over to the longitudinal vein caudad of the 

 second media, and continues its course to the margin of the wing (Fig. 

 74). The point of separation of the radial sector from the second 

 media is not evident, there being no oblique cross-vein as in the Ani- 

 soptera. The vein uniting the caudal end of the cross-vein over which 

 the radial sector crosses, to the main radio-medial trunk is known as 

 the bridge (seen in Lestes and Ischnura, Figs. 81, 85; br), and is 

 secondary in origin. In Hetaerina the trachea of the bridge is fully 

 as strongly developed in the nymph as any other of the main tracheal 

 trunks. Such a feature would perhaps throw some doubt on the actual 

 formation of the bridge in this suborder were it not for the strong 

 comparative evidence present in the Anisoptera. The bridge reaches 

 R-plus-M about half-way between the nodus and the base of the wing. 

 About one-third of the distance from the base to the nodus is a strong, 

 oblique cross-vein, the arculus (Figs. 81, 85; arc), from the middle 

 of which two longitudinal veins arise. These veins are the third and 

 fourth median veins (Mg and M4), respectively, the cephalic one being 

 Mg. A short distance from the arculus there is another heavy cross- 

 vein connecting M4 with the longitudinal vein caudad of it. The cross- 

 vein probably represents the medio-cubital cross-vein. The four-sided 

 area enclosed by this vein, the portions of M4 and the longitudinal vein 

 caudad of it (the cubitus) and distad of the arculus, forms what is 

 known as the quadrangle (qd), and corresponds to the cell first M4. 

 The cubitus extends from the base of the wing to the distal side of the 

 quadrangle, where it forks and sends out two longitudinal branches 

 caudo-laterad to the margin of the wing. The forks are Cu, and Cu,, 

 or first and second cubitus. The anal vein (A) consists of a single 

 heavy trunk extending from the base of the wing and apparently con- 

 necting with the cubitus at the point where the latter forks. The dif- 

 ferent anal veins can not be traced because of numerous secondary 

 cross-veins. 



Many variations occur in the above wing-venation, but instead of 

 a discussion of each in detail the reader is referred to figures 73 and 

 81-90 which show the types of venation occurring in the remaining 

 genera of Zygoptera found in Illinois. 



Abdomen (Figs. 91-100, 104). — The abdomen of all Zygoptera is 

 cylindrical and composed of ten complete segments. In all of the seg- 

 ments the sterna are much reduced and hidden by the overlapping 

 terga. The pleura are still more reduced, so that no portion of them 



