IX] THE RESPIRATORY SYSTEM 177 



come off, each of which soon branches to supply the median 

 ocellus, and the lateral ocellus of its own side. Thus the median 

 ocellus receives a double tracheal supply. The lower branches of 

 the dorsal trunks divide into two branches below the bases of the 

 arch. The more posterior of these two branches is very thick. 

 It soon divides up in a fan-like manner to supply the optic 

 ganglion. The more anterior branch is slenderer. It runs forwards 

 in a double curve to enter the antenna, giving off branches to the 

 brain, the mandibles and maxillae, and the frontal region, on 

 the way. 



The above description was made from a nearly full-grown larva 

 of Synlestes (fig. 77). In Aeschna, the superior anastomosis is 

 comparatively weak, but the other main branches are much 

 stouter than in Synlestes. There is no branch corresponding to 

 the one from the arch to the anterior part of the eye. The branches 

 to the optic ganglion and the brain are united for a short distance. 

 They then separate, and spread out as two immense fans, deeply 

 coloured with brown pigment. 



Modifications in the Spiracles. The mesostigma in the larva 

 is well formed and easily seen. It is shorter and broader than in 

 the imago, somewhat resembling the metastigma of the latter, 

 though not so rounded. It is open, and provided with a row of 

 bristles beneath one lip. It is clear that this spiracle can become 

 functional whenever the larva has need to leave the water for a 

 time. Many Aeschnine larvae wander about on rocks or other 

 damp places at night, using these spiracles for breathing. The 

 metastigma is very small, being little more than a point. In the 

 Anisoptera it is just open, by a slit; in the Zygoptera, it seems 

 to be quite closed. With few exceptions, it appears to be non- 

 functional. The abdominal spiracles have been already mentioned. 



An excellent account of the thoracic spiracles and abdominal 

 tracheation of Anisopterid larvae has been given by Oustalet 1 [Hi]. 

 I know of no detailed account of the tracheae in the imago. An 

 excellent short account is, however, given by Calvert [22]. 



1 He, however, denies the existence of abdominal spiracles. 



T. D.-F. 12 



