TRACHEAE IN THE NYMPH OF PLATHEM1S I.YDIA. 349 



trachea then just back of the mesothoracic ganglion it receives 

 two branches, one from the mesothoracic branch (Fig. 2, MSB) 

 and the other from the mesothoracic trachea itself (Fig. 2, MS 

 TH). Finally the neural trachea receives two branches pos- 

 teriorly. The first arises from the metathoracic branch (Fig. 2, 

 M T B) the second from the metathoracic trachea (Fig. 2, MT 

 777). In addition the metathoracic ganglion as previously de- 

 scribed receives fine tubes from the tracheae which supply the 

 first abdominal ganglion. 



From the dorsal trachea posterior to the metathoracic there 

 arise four tubes connecting the dorsal with the lateral trachea 

 (Fig. i, D L C). The lateral tracheae at the anterior end are 

 not much larger than these dorsolateral connectives. 



Rectal brancliice. Like many of the larval dragon flies Plathe- 

 inis lydia takes in air from the water which passes in and out of the 

 rectum. The rectum of the nymph is proportionately much larger 

 than in the adult. It is supplied with a complicated apparatus for 

 separating the air from the water. The posterior end of the ab- 

 domen at the anus is provided with anal spines. There are three 

 larger spines and a pair of smaller ones. Of the larger ones, two 

 are ventral, one is dorsal while there is a single smaller spine be- 

 tween the dorsal and each of the ventrals. Just internal to these 

 spines are three valves each adjacent to 



one of the larger rectal spines. The /^-'"""^xv^ R V 

 dorsal valve (Fig. 3, D R V)\s the larg- 

 est, the two ventral valves (Fig. 3, V R 

 V) are of equal size.. Each edge of the 

 dorsal valve is met by the external edges 

 of the other two, a triradiate slit being 

 thus formed when the valves are closed. 



The ventral valves seem to work on the dorsal valve as a base. 

 When the valves close water is retained in the rectum. It is forced 

 out again by the compression of the abdomen. If it is ejected 

 with sufficient force the animal is propelled forward in the water. 



This compression of the abdomen is brought about by means 

 of muscles connecting the roof and floor of the abdomen at its 

 external edges. Expansion of the abdomen is due to two causes, 

 first the relaxation of the above mentioned abdominal muscles 



