23 



ages. These are of considerable value in classification. The 

 posterior lateral angles of some of the abdominal segments bear 

 the lateral spines, sometimes very small but distinct, as in Li- 

 heUula, rarely entirely wanting as in MesofJiemis. 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 Juwks. 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 Go}}i- 

 plms, 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 imj)ortance, 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 jEschnidee, 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 Ani- 

 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 



