116 



RESPIRATORY MECHANISMS 



A 42 



Fig. 65. Structure of tracheae. A, 

 close to spiracle. B, small branch more 

 highly magnified, a epicuticle with 

 spiral folds, d epithelial matrix. 

 (Weber.) 



of tracheae branch out to the organs in the corresponding 

 segment or segments. The spiracles are connected lengthwise 

 by tracheal tubes, but these are of secondary importance only. 

 The tracheae are circular in cross section with chitinous walls 

 fortified by a spiral fold as shown in Fig. 65. They branch 



fairly regularly and the ag- 

 gregate cross section remains 

 very nearly constant in the 

 repeated branchings. The 

 finest chitinous branches are 

 continued in extremely nu- 

 merous and narrow "trach- 

 eoles" which are non-chitin- 

 ous. All the tracheae are 

 very resistant against pres- 

 sure, which will produce only 

 a slight shortening of the 

 stems and branches. In 

 these animals there are no 

 specific respiratory movements, and the pressures set up in the 

 body fluids by locomotion or even by violent struggling can 

 cause ventilations only which are too small to be significant. 

 When the average length of the tracheae and their aggregate 

 cross section is made out it becomes possible to calculate the 

 pressure difference necessary for a certain amount of oxygen, 

 corresponding to the metabolism of the animal, to diffuse from 

 the atmosphere to the tracheoles. Measurements made on 

 the tracheal system of a large Cossus larva gave an aggregate 

 cross-sectional area of all the tracheae supplying the tissues of 

 6.7 mm 2 with an average length of 6 mm. Through a tube 

 of these dimensions the oxygen necessary for the animal's 

 metabolism, viz., 0.3 mm 3 /second, will diffuse by a pressure 

 difference of 11 mm, 1 which means that diffusion is ample to 



1 The diffusion rate for CO2 in air is slightly lower, and the same quantity 

 would require a pressure difference of 13 mm. The CO2 produced is on an 

 average somewhat less than the oxygen used up, and 10 to 25% are eliminated 

 through the skin. The actual pressure difference in the tracheal system neces- 

 sary for the transport of CO2 is therefore rather lower than that for O-. 



