35 2 G. G. SCOTT. 



Oustelet and Palmen state that this rectal breathing apparatus 

 does not disappear at the last ecdysis of the nymph but remains, 

 though functionless. Hagen says that in a form he studied 

 the whole structure disappears at that time. Whatever be the 

 case, as far as the final moult is concerned, it is true that in the 

 adult Platlicmis lydia the rectal breathing apparatus has disap- 

 peared. There is indeed no further use for it since the stigmata 

 are then functional. 



The tracheal system in the nymph of Plathemis lydia differs in 

 most of its details from that of SEscJina cyanea described by 

 Oustelet, '69. He describes four main trunks extending the 

 length of the body and a pair of smaller abdominal tubes. 

 These correspond to the dorsals, ventrals and laterals in Plath- 

 emis lydia. The four main trunks supply the rectum and respir- 

 atory papillae in a manner roughly similar to that in Plathemis 

 lydia. In sEschna cyanea, however, the rectal tracheae arise 

 from the hinder part of the dorsal at regular intervals, while in 

 Plathemis lydia the hind part of the dorsal abruptly breaks up 

 into many fine branches. The rectum of the former is furnished 

 with respiratory papillae, while the latter has flattened leaf-like 

 gills. In both the tracheoles are present in loops. 



In agrionid nymphs there are three long, flat caudal processes 

 thickly supplied with tracheae, and serve as an apparatus for 

 taking the air from the water into the tracheal system. They 

 are morphologically identical with the caudal spines in P. lydia. 

 In the nymph of Caloptcryx, an agrionid, there are a few internal 

 rectal gills in addition to the three long caudal gill processes. 

 In these forms the gill apparatus is mostly exterior. In the 

 yEschnidae the branchial apparatus is within the rectum. The 

 inside surface of the rectum is provided with six longitudinal 

 bands, each bearing a double row of folds or papillae. In the 

 Libellulidae, 'of which PlatJicmis lydia is a member, we find not 

 papillae but flat processes or gills. 



In the Libellulidae there is found the highest form of the rectal 

 gill breathing apparatus, the most specialized branchial arrange- 

 ment of these three groups. On the other hand the Agrionidae 

 have the simplest form while the /Eschnidae have the form of 

 branchiae intermediate between the other two. If we were to 



