The Tracheal System of Simulium. 



705 



spiracle-chamber is connected with the longitudinal trunk 

 by means of a short segmental branch, and the following 

 branches lead from it : (1) a longitudinal commissure with 

 the mesothoracic 

 spiracle - chamber 

 (fig. 8, c) ; about 

 the middle of its 

 course it gives off a 

 trachea which runs 

 iu the sheath of 

 the mesothoracic 

 leg ; (2) an ascend- 

 ing trachea which 

 divides into a brush 

 of tracheolas ; (8) a 

 descending trachea 

 which forks ; (4) a 

 trachea to the 

 sheath of the meta- 

 thoracic leg. 



The mesothoracic 

 spiracle can be 

 found most readily 

 when the larval 

 skin is being dis- 

 engaged from the 

 body of the young 

 pupa. The old 

 tracheae of the head 

 and part of the 

 thorax are then 

 withdrawn through 

 the spiracle in 

 question. It is 

 situated a short 

 distance behind the 

 base of the gill. 

 It becomes closed 

 when the trachese 

 are cast and the 

 initial thread be- 

 comes solid. The spiracle-chamber gives off several tracheal 

 trunks. Of these the longitudinal trunk and the commissure 

 with the metathoracic spiracle-chamber have been already 



