451 



always divided in the nymph as in the adult, but seems to be simpler 

 in structure in the nymph. The mesostigmal plates of Zygoptera are 

 not developed until the adult stage, but the depressed area caudad of 

 the mesoscutum in the Anisoptera is frequently present in the 

 nymphal stages, especially in the Libellulidae. The nymph molts 

 several times after eclosion before the rudiments of the wings appear 

 as minute ridges on the dorsum of the mesothorax and metathorax. 

 They develop subsequently like the wings of heterometabolous insects 

 in general. As already noted, the crossing of the radial sector over 

 the media can not be followed, and in only one genus, Lestes, is there 

 any recognizable portion of the radial sector. The character of the 

 tracheation of the wing-cases of several zygopterous nymphs is 

 shown in Figures 14-17. 



Abdomen. — Very little can be said of the abdomen except tliat in 

 both the embryo and nymph the segments are about equal in length 

 and more or less cylindrical. Reduction in size, lengthening of the 

 segments, and flattening of the abdomen, together with the appear- 

 ance of dorsal and lateral spines, seem to be the developmental ten- 

 dencies in the nympli. The accessory genitalia of the adult show no 

 signs of development until near the last nymphal stage, but the ovi- 

 positor of the female appears early, at least in the Zygoptera. This 

 organ undergoes great modifications and specialization in the adult 

 Zj'goptera, but in the Anisoptera it is probably in the process of re- 

 duction and degeneration. The caudal tracheal gills of the Zygoptera 

 are present in the embryo, and at hatching they appear as cylindrical, 

 jointed, cerciform appendages. Brandt ('69) says that at a still 

 earlier stage the lateral pair of gills are fused, but this observation 

 has not been verified. There is also a pair of smaller cerci dorso- 

 laterad of the lateral gills, making five caudal abdominal appendages 

 in all. All five of these are represented in the Anisoptera by short 

 cerciform appendages which are frequently triquetal and often 

 sharply pointed at the apex. It is important to note that these ap- 

 pendages are never united in the nymphs of Anisoptera or in Zygop- 

 tera, but that in the adults of Anisoptera the ventral pair is sometimes 

 fused. In all families of Zygoptera, the superior abdominal ap- 

 pendages, 'which replace the lateral gills, are greatly reduced, but in 

 some Anisoptera, family Aeshnidae, the lateral appendages are re- 

 placed in the adult by long, lateral, superior appendages resembling 

 gills. A fact which sheds light on the origin of the Odonata as a 

 whole, is the presence of lateral abdominal gills in the genus Cora of 

 Central America and Euphea of the Old World. The rectal gills of 

 Anisoptera have been thought to originate in the forms having tra- 

 cheae which anastomose on entering the walls of the rectum as in 



