140 



RESPIRATORY MECHANISMS 



by a very dense system of fine tracheae arranged just below the 

 cuticle. When a difference is established between the oxygen 

 pressure in these tracheae and the tension in the surrounding 

 water, oxygen will diffuse in from the water and become dis- 

 tributed by diffusion within the tracheal system to all parts 

 of the body. 



Considered simply as gills, the tracheal gills 

 of insect larvae duplicate practically all the 

 gill arrangements observed in combination 

 with a circulation, except those of fishes. We 

 find cases like the one shown in Fig. 81 in 

 which there is no branchial enlargement, but 

 only a system of tracheae below the cuticle. 

 We find many cases of feathery or leaf-like 

 tracheal gills differently arranged (Plecoptera, 

 Ephemeridtz, Trichoptera, ^ygoptera, Gyrinidce, 

 and others). 



In some of these the gills are immobile and 

 the renewal of water takes place by the loco- 

 motion of the animal. In others the gills 

 themselves can be moved. In the Trichop- 

 tera larvae inhabiting tubes we find the tube 

 ventilated by suitable movements of the larvae 

 just as are the tubes of many Annelida. 



We find finally in the nymphs of the large 

 dragon flies (Anisoptera) a system of tracheal 

 gills in the rectum presenting a very large sur- 

 face (Fig. 82) and very richly supplied with 

 tracheae (Fig. 83). On Aeschna nymphs Koch 

 (1936) measured the gill area and found it to 

 be 7 cm 2 corresponding to an external surface of 3 cm 2 . The 

 gill system in these forms is curiously analogous to the gills 

 of the cuttle fish (.p. 38) and likewise ventilated by powerful 

 muscular movements which may serve simultaneously for the 

 locomotion of the animal and thus provide a close connection 

 between the call for oxygen as determined by activity and the 

 supply of oxygen to the gills. 



Fig. 81. First 

 stage larva of 

 Macrocentrus giju- 

 ensis. Ichneu- 

 monidce; lateral 

 view, to show 

 the rich tracheal 

 supply beneath 

 the skin. (Par- 

 ker.) 



