126 



RESPIRATORY MECHANISMS 



making up the vital capacity or two-thirds of the total is 

 evidently completely renewed, but in the remaining third part 

 renewal can take place only by diffusion in the narrow, but 



numerous, circular tracheal 

 branches. 



The ventilation tracheae 

 in insects generally are either 

 tubular with an elliptic cross 

 section as in the example 

 discussed above, or they 

 have the form of air-sacs 

 which can be of very vary- 

 ing form and shape from 

 very numerous small vesi- 

 cles (in the cockchafer) to 

 very large structures like 

 those shown in Fig. 71. In 

 all cases, however, the final 

 branches are circular in cross 

 section and allow only diffu- 

 sion transport. This is why 

 even with mechanical ven- 

 tilation of the air-sacs or 

 tracheal trunks the size of 

 insects must be limited by the 

 amount of oxygen which can diffuse into their tissues. This 

 limit was probably reached by the dragon-fly-like Meganeura 

 of the Carbon period, which reached a length of over 30 cm, 

 being 3 cm broad across the thorax. 1 



The ventilation usually takes place by movements of the 

 abdomen either in the dorso-ventral direction or by telescoping 

 the segments. In a few cases (Dytiscus and Hydrophilus, ac- 

 cording to Brocher, 1931) there are respiratory movements 



1 Certain large air-sacs have very little to do with respiration, but have as 

 their main function to take up space which in the rigid body of the insect may 

 later be required for other purposes. This is the case with the two large proximal 

 air-sacs in the abdomen of flies according to Evans (1935) and Fraenkel (1935). 

 Their role is well illustrated by Fig. 72 after Evans. 



Fig. 71. Tracheal system in the 

 abdomen of the honey-bee worker. 

 Dorsal tracheae and air-sacs have been 

 removed. (Snodgrass.) 



