IX 



Tracheal System, Endoskeleton and Hypodermis 



i. Tracheal System 



The essential features of both the structure and development 

 of the tracheal system have been correctly described by both 

 Butschli (1870) and Grassi (1884). The tracheal system of the 

 larva consists of a pair of longitudinal trunks (Figs. 63, 64 and 

 75, TraTr, Fig. XV, LTraT), each of which traverses virtually 

 the entire length of the trunk close beneath the lateral hypoder- 

 mis. These trunks are connected to the ten spiracles (Sp) of 

 each side by slender short branches. At their anterior and pos- 

 terior ends, respectively, the two lateral trunks are united by 

 semicircular loops or commissures, one of which is situated ceph- 

 alad of the mid-intestine, above the oesophagus (Figs. 63 and 

 64, ATraL, Fig. XV), the other caudad of the mid-intestine, 

 just ventrad of its juncture with the hind-intestine (Fig. 63 

 PTraL, Fig. XV). The tracheal trunks are also united between 

 these points by eleven other commissures, segmentally arranged. 

 These commissures arise from the ventral side of the lateral 

 trunks, nearly opposite the point of origin of the branches to 

 the stigmata, thence they pass downward, close to the hypoder- 

 mis and beneath the ventral cord (Figs. 63 and 64, TraCom, 

 Fig. XV). The first and second of these loops join the main 

 trunk close to the anterior boundary of the second and third 

 trunk segments, and intersect the ventral mid-line between the 

 first and second trunk segments. The third and fourth loops 

 are united at their bases, joining the main trunk in the anterior 

 half of the fourth trunk segment. These loops intersect the 

 ventral mid-line near the posterior margin of the third and 

 fourth trunk segments. The remaining seven loops are con- 

 tained entirely within the limits of the corresponding segments. 

 Just in front of each of the spiracles the tracheal trunk sends 

 off one or more branches which pass dorsad, breaking up into 

 finer branches supplying the dorsal region of the trunk. Small 



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