25 
principal ones may usually be observed in the hind wing. The 
Jirst branch 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 anal loop. This and the triangle are particularly 
useful in classification. 
The male imago may be 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 Libellulide 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 Zygoptera.) 
bb. Basal segment of the antenne extremely elongate. 
Calopterygide. 
b. Basal segment of the antenne short, subrotund. 
Agrionide. 
a. Last abdominal segment terminating in five converging, 
short, spine-like appendages. (Suborder An/soptera.) 
cc, Labium flat, not concealing the face, with mandible-like 
or oblong lateral lobes. 
dd. Antenne 6- or 7-jointed, superior appendage usually 
notched at apex. 
ee. Lateral labial lobes toothed on terminal margin, 
antenne broad, flattened. Petaluride. 
e. Lateral labial lobes toothed only on the inner edge, 
antenne slender. AE'schnide. 
