25 



principal ones may usually be observed in the hind wing. The 

 first h ranch of the anal vein starts near or at the posterior cor- 

 ner of the triangle, and the other two branches are between 

 that and the base of the wing. The first and second branches, 

 in the hind wing, are connected by a strong cross-vein, thus 

 inclosing the ancd loop. This and the triangle are particularly 

 useful in classification. 



The male imago may l)e recognized by the complex struct- 

 ure on the under side of the abdomen near its base, forming a 

 conspicuous projection. In the families treated in this article 

 the male has the anal angle of the hind wing sharply rectangu- 

 lar, not rounded as in the female. In the LihelluJidie it may 

 sometimes be known by the appearance of milky spots on the 

 wings, in addition to the usual black spots, or by a difference in 

 wing-markings. The terminal appendages of the male are un- 

 like those of the female, being usually larger and more irregu- 

 larly formed. 



KEY TO THE NORTH AMERICAN FAMILIES OF ODONATA. 



Nymphs. 



aa. Last abdominal segment bearing three leaf -like tracheal 

 gills. (Suborder Zipjoptera.) 

 hb. Basal segment of the antennae extremely elongate. 



Calopteryijida'. 

 b. Basal segment of the antennae short, subrotund. 



Afjr ion idle, 

 a. Last abdominal segment terminating in five converging, 

 short, spine-like appendages. (Suborder Anisoptera.) 

 cc. Labium flat, not concealing the face, with mandible-like 

 or oblong lateral lobes. 

 dd. Antennae 6- or 7-jointed, superior appendage usually 

 notched at apex. 

 ee. Lateral labial lobes toothed on terminal margin, 

 antennae broad, flattened. Petaluridtc. 



e. Lateral labial lobes toothed only on the inner edge, 

 antennae slender. yEschnidte. 



