NORTH AMERICAN ODONATA. 199 



corresponding to the dorsal gill. In the males of the Anisoptera 

 the dorsal appendage of the nyn)ph is represented by the single in- 

 ferior appendage of the imago, and the two lateral appendages by 

 the right and left halves of the tubercle in which the anus lies, below 

 the inferior appendage. The inferior appendages of the males of 

 the Zygoptera and Anisoptera are therefore not homologous. 



The only external genital organs which appear in the nymphal 

 period are to be found on the ventral surface of the ninth segment 

 of older nymphs of the Agrionidse, the Aeschninse and the Cordule- 

 gasterin^e. The female nymphs of these groups may be recognized 

 by the longitudinal ridges, which correspond to the form of the vul- 

 var lamina and the genital valves of the imago. The old male 

 nymphs of the Agrioninse have two ventral hooks near the apex of 

 9, representing the two valves closing the orifice of the common duct 

 of the testes. The same are also indicated by a slight eminence in 

 male Aeschnine nymphs. 



The information which exists as to the development of the internal 

 organs is very little. The abdominal muscles form a more complete 

 sheath than in the imago ; they are well developed in the youngest 

 nymphs. Poletaiew (5) believes the wing-muscles to be formed at 

 the same time that the wing-covers first appear ; they then are com- 

 posed of globules similar to those of adipose tissue and interlaced 

 with trachese, while all the other muscles are striated, as are the wing 

 and remaining muscles of the imago. In consequence of the shape 

 of the thorax, the wing-muscles of the nymph are more vertically 

 placed at first, and subsequently change their direction in conformity 

 with the increasing obliquity of the thorax. 



The positions of the three parts of the alimentary canal correspond 

 in older nymphs at least, to those of the imago, and the histological 

 structure of the mid-gut is the same (Faussek 7). The characteristic 

 features of the hind-gut will be referred to in connection with the 

 respiratory apparatus. The time of the first appearance of the Mal- 

 pighian tubules is unknown, but they are well developed in old 

 nymphs. 



The res'piratory apparatus has been carefully studied by Oustalet 

 (28) and Roster (30) in the nymph of the European Aeschna cyanea 

 {maculatissima) and by Miss Olga Poletaiew (29) in those of Ae. 

 (jrandis and juncea. Oustalet distinguishes three main pairs of tra- 

 cheal trunks — dorsal, viseral and ventral, as already described for 

 the imago of Libellula pulchella. The dorsal pair lie on the dorsal 



TKAN8. AM. ENT. SOC. XX. AUGUST, 1893. 



