IX] THE RESPIRATORY SYSTEM 171 



with the abdominal spiracles, but each gives off six cross-branches 

 to the corresponding ventral trunk, close to the levels of the second 

 to the seventh of these spiracles. On entering the thorax, the 

 dorsal trunks dip ventrally downwards, and run forwards to 

 below the mesostigmata, from which they receive short strong 

 branches. They also receive longer branches from the meta- 

 stigmata. On entering the head, each divides into two. As the 

 tracheal supply of the head is practically the same in imago and 

 larva, and as it can only be followed satisfactorily in the latter, 

 we have postponed a general account of this part to the next 

 section. 



Each dorsal trunk gives off the following principal branches : 



(i) Branches to the three legs (pedal tracheae). 



(ii) Branches to the two wings (alar trunks, best developed in 

 the larva). 



(iii) Branches to the thoracic muscles. 



(iv) Short branches with blind ends, capable of being distended 

 into air-sacs (as). These air-sacs are of very irregular occurrence 

 (i.e. they are probably not noticed unless they are distended). 

 Usually at least two pairs can be found in the second segment, 

 lying on either side of the usually more distended crop. I have 

 also found small air-sacs in other segments, particularly from the 

 sixth to the eighth. Along this region, the dorsal trunk gives off 

 branches which ordinarily -lie close alongside the main trunk, 

 directed backwards. In some cases the posterior ends are found 

 to be distended, forming long sausage-shaped air-sacs. I have 

 noticed this particularly in Orthetrum and Hemicordulia, where 

 the abdomen is dilated. 



(v) The six cross-branches to the ventral trunks, already 

 mentioned. The most posterior of these is very slender, the others 

 strong and thick. 



(vi) Numerous branches to the heart, particularly along the 

 anterior half. These branches, on reaching the heart, bifurcate 

 into two widely diverging portions, one running anteriorly, the 

 other posteriorly. From each of these, numerous smaller branches 

 arise, so that the heart is covered by a complete net-work of 

 tracheae. 



At its posterior end, each dorsal trunk turns inward to enter 



