NORTH AMKRICAN ODONATA. 169 



Post-cab Uals { post-nodals Kirby) are cross-veins between the costa 

 and the j)rincipal sector, and the nodus and the pterostigma, and of 

 which there are similarly two series. 



Basilars, supra-triangidars and median cross-veins cross respectively 

 the basilar, supra-triangular and median spaces. The basal post- 

 costal vein (Selys) in the Agrionime is a median cross-vein. Subcostal 

 cross-veins is the term applied by Karsch to cross-veins confined to 

 the space between subcostal and median veins and on the basal side 

 of the first antecubital. 



In addition to the veins and sectors which have already been de- 

 scribed, there exist between them near the hind margin of the wing, 

 varying numbers of more or less parallel supplementary, interposed 

 sectors. Of these the chief one between the princ pal and nodal 

 sectors has received the name of ultra- or post-nodal. 



Eedtenbacher, in his paper on the wing-veins of insects (Ann. k. k. Natiirhist. 

 Hofmus. Wien I, pp. 153-231, 1886), numbers the veins of the Odonate wing as 

 follows: I costa, II subcosta, IIIi median vein, III2 principal sector, III3 ultra- 

 nodal sector, IV nodal sector, V subnodal sector, VI median sector, VII short 

 sector, VIII upper sector of the triangle, IX lower sector of the triangle. 



While the anal margin of the hind wings is usually convex and 

 the anal " angle" rounded, in the males of many Aeschnidie and of 

 the Cordulinae the anal margin is concave, and the angle present. 

 De Selys has shown that this concavity is correlated with the pres- 

 ence of a denticulated tubercle (auricle) on either side of the second 

 abdominal segment, and suggests that the hollowing out of the anal 

 margin prevents tearing the wing on the auricular teeth, as would 

 be the result were the anal margin convex. No such concavity of 

 the anal margin is present in the corresponding females, as even 

 when auricles occur in this sex, they are much smaller and not den- 

 ticulated. 



The abdomen consists of ten distinct segments, and a terminal 

 tubercle with the anal opening (anal segment, telson). It is conve- 

 nient to designate the abdominal segments from base to apex, by the 

 figures 1 to 10; of these 1 is always the shortest. The exoskeleton 

 of each segment coufists of a large dorsal sclerite (tergum), forming 

 an arc much greater than 180°, which overlaps and partly hides the 

 much smaller sternum. Tergum and sternum are connected bv a 

 pleural membrane on either side, which allows of the distension and 

 contraction of the segment in the act of respiration. In most of the 

 groups a longitudinal ventral carina is present near each pleural 



TEANS. AM. ENT. SOC. XX. (22) JULY. 1893. 



