March, i 9 1 9-1 VlNAL : CAROLINA LOCUST. 25 



3) and by removing the external chitin of the thorax (Plate IV, 

 Fig- 7)- 



In general, tracheal tubes originate at the lower or smaller cham- 

 ber of the first thoracic spiracle and pass backward between the 

 muscle layers to the first abdominal spiracle (///) and connect with 

 the second spiracular plexus at the different intersections of this 

 plexus and the muscle layers. 



The tracheal system of the thorax is best understood if two divi- 

 sions are discussed separately: (i) Dorsal tracheae which supply air 

 to most of the thoracic muscles, and (2) ventral tracheae which 

 supply the head, nervous system and legs. 



Dorsal Thoracic Trachea. — The median sagittal section and its 

 air tubes will be discussed first (Plate III, Fig. i). From spiracle III 

 a branch is given off which runs to the median surface of the dorsal 

 longitudinal muscles (III-Ba), where it divides into three branches 

 which distribute themselves over the surface of these muscles, but all 

 connect anteriorly with the main thoracic air sac B while still in the 

 metathorax. This is the largest air sac found in the body and runs 

 contiguous to the median surface of the dorsal longitudinal muscles, 

 giving off many branches to these muscles. Just below the phragma, 

 dividing the meta- and meso-thorax, this important air sac gives off 

 a branch, which runs transversely beneath the dorsal longitudinal 

 muscles and forms a compound plexus (II-p), receiving branches of 

 the thoracic tracheal system at the different muscle levels, and finally 

 connecting with the second thoracic spiracle. Anteriorly another 

 ventral branch is given off by air sac B which joins indirectly the 

 smaller chamber of spiracle /. This completes the dorsal thoracic 

 tracheae shown in Fig. i. 



On the lateral surface of the dorsal longitudinal muscles two 

 arched tracheae are found supplying air to these muscles. These are 

 only partly shown in Plate III, Fig. 2. One of these originates at spi- 

 racle / (I-D) and connects posteriorly with air sac D, situated just 

 anterior to the phragma dividing the meso- and meta-thorax, which 

 in turn is joined ventrally with plexus II-p. The lateral side of the 

 longitudinal muscles of the metathorax is furnished by an arched 

 tracheal tube which originates at II-p (Fig. 2, 1 1 -1 1 1 a) and ends at 

 spiracle ///. 



