Jan., 1922 the tracheal branches of insects 85 



The nymph of Lestes, which furnished the material for this 

 study, is so abundant and so transparent that little dissection 

 was needed. 



In this article the writer will outline the homologies of the 

 principal thoracic tracheal branches as referred back to the 

 simpler system of an individual abdominal segment. The terms 

 used are descriptive as far as possible and were worked out 

 with the help of Dr. Chapman with the hope that we had terms 

 that would be useful in future tracheal studies. 



If the reader will refer to the plate he will see in Fig. 1 the 

 simplest tracheal unit in abdominal segment 2. It is repeated 

 in segments 3-7. Segments 1 and 8-10 have more or less mod- 

 ified versions of this same unit. In the thorax the tracheal 

 branches are yet more highly modified. The diagram Fig. 3, 

 shows what the writer considers as the hypothetical primitive 

 unit from which the present system with its longitudinal trunks 

 has been developed. 



The studies of the primitive tracheal systems in Peripatus 

 and in the embryology of the insectean tracheal system show 

 that the spiracles develop first as pits. From the bases of these 

 pits the tracheal branches develop. Our unit starts then with 

 (1) the spiracle, sp., extending into the body wall, (2) is the 

 spiracular pit, spp. From the bottom of this there develops 

 dorsad a short stout branch, (3) the spiracular trunk, spt. 

 This trunk forks and sends a branch, (4) the anterior dorsal 

 connective, adc. to the muscles of the anterior end of the seg- 

 ment, and a branch (5) the posterior dorsal connective, pdc. to 

 the posterior muscles of the dorsum of the segment. Each of 

 these has a branched tip supplying its group of muscles, (6) (7) 

 the tip of the anterior dorsal connective, tadc, and the tip of 

 the posterior dorsal connective, tpdc. These tips become land- 

 marks later in the study. 



The spiracular pit sends out three other branches, (8) the 

 ■anterior spiracular connective, asc, which connects forward to 

 the leg trachea and which gives off a small branch to the body 

 wall, (9) the posterior latero-tergal trachea, pit. Opposite to this 

 (8) is given ofif a larger trachea, (10) the leg trachea. It, which 

 runs caudad and ventrad. It gives off first, (11) the anterior 

 latero-tergal trachea, alt, to the body wall, then (12) the pleural 

 trachea, pt, to the pleural fold. Ventrad the spiracular pit gives 



