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ages. These are of considerable value in classification. The 
posterior lateral angles of some of the abdominal segments bear 
the /ateral spines, sometimes very small but distinct, as in L/- 
bellula, rarely entirely wanting asin Mesothemis. There are also 
spines or teeth along the median line of the abdomen above, 
varying greatly in number and form, collectively known as the 
dorsal hooks. These and the lateral spines are extremely use- 
ful in separating species and subordinate groups. The dorsal 
hooks may be viewed in profile from one side, springing the 
abdomen down away from the wing-pads ; but when they are 
represented merely by teeth projecting backward on the hind 
margin and not elevated above the general level, as in Gom- 
phus, they are of course best viewed from above. The dorsal 
hooks are always absent on the first one or two abdominal seg- 
ments, beneath the wing-pads, and as their appearance on the 
posterior abdominal segments is of especial importance, it has 
usually been necessary to refer only to those on the exposed 
segments of the abdomen. 
The characters given can usually be applied to nymphs of 
any age except the very young ones. In case more or less of 
an antenna or leg is broken off during the life of a nymph, it 
may be imperfectly replaced, usually with one joint less than 
before and the relative size of the joints abnormal. 
A slight elevation on the under side of the abdomen, if near 
its base, indicates the male nymph ; if towards its tip, the fe- 
male. This may be easily observed in the common nymphs of 
Anax junius. In the A’schnide, the male nymph is also dis- 
tinguished by what seems to be a small median scale resting on 
the basal part of the superior appendage. 
The Imago.—It is unnecessary here to describe in detail 
the external anatomy of the adult, but for the understanding 
of the keys a brief statement of the wing venation in the Avni- 
soptera is desirable. The fore and hind wings have essentially 
the same structure, modified to suit the difference in outline 
especially at the anal angle. The large number of adventitious 
minor longitudinal veins and cross-veins and cells are extremely 
