TEACHER. 



of the body (fig. 66 a, b) (stigmata of Insects, Spiders), or they 

 may be more restricted in number, and communicate also with 

 cavities of complicated structure which are used for other functions 

 (nasal cavities of Vertebrates). In the aquatic larvae of certain 



Fro. 67a.- -Larva of an Ephemeral fly \vith seven pairs of trachea! gflis 

 -T/, slightly magnified ; Tk, isolated trachea! gill strongly magnified. 



/la/ 



FIG. 676. Tracheal sys- 

 tem at the sides of the 

 alimentary canal of 

 an Agrion larva (after 

 L. Dufour). Tst, main 

 tracheal trunk ; JTf, 

 tracheal gills ; Na, the 

 three simple eyes. 



Insects (Ephemeridse, Libellulidae) the tracheae may be without any 

 external openings. In such cases processes of the body filled with 

 a close network of trachese, which take up oxygen from the water, 

 and are known as tracheal gills, are developed (fig. 67 a, b). In rare 

 instances tracheal gills are developed on the wall of the rectum, and 



