SOUTH AriUCAN TERMITES. 91 



epiti-achelos is removed to show (in solid black) the side opening in 

 the organ through which the air passes into the epiti'achelos. In 23c 

 the epitrachelos is seen closed by the pinching of tlie lever against the 

 resister (more or less diagi-ammatic). 



Figs. 24a and 24b. — The abdominl spiracle in the second stage of 

 development (T. natalensis). Fig. 24a, viewed from within ; fig. 24b, 

 the outer aspect. Both figures illustrate the development of the resister 

 and its hiatus, the extension of the trachelos, and the disappeai-ance of 

 the epitrachelos. 



Figs. 25a, b, c. — Sections of the lever, showing the hypothetical 

 development or change from an ascidium to a plate. (Diagram s.) 



Fig. 26a. — A section of the base of the atrium, showing how, in the 

 second stage spiracle, the resister protrudes into the lumen. (Dia- 

 gram.) 



Fig. 26b. — An interior view of the resister, showing the hiatus and 

 bulge. (Diagram.) 



Figs. 27a and b. — ^The resister and lever, showing dorsal strap or 

 fulcrum connecting the two ; fig. 27a, viewed from within ; fig. 27b, the 

 outer aspect. (Diagrams.) 



Figs. 28a and b. — The resister and lever with muscle attached ; 

 fig. 28a, attitude when the lumen is open ; fig. 28b, when the lumen is 

 closed. (Diagrams.) 



Figs. 29a and b. — Dorsal aspect of spiracles ; fig. 29a, showing the 

 lever drawn against the resister and the lumen closed ; fig. 29b, showing 

 the relationship of lever to resister when the lumen is open. (Dia- 

 grams.) 



Fig. 30. — Abdominal spiracle in transition lietween second and third 

 stages (T. natalensis). View from within, showing nr., a new rim 

 developing behind the rim of the pore (p>-.).and ch., the large chin-plate 

 that develops on the side of the atrium against the resister. 



Figs. 31a, b, c. — Abdominal spiracle of the third stage (T. nata- 

 lensis). Fig. 31a, viewed from within, showing (1) the shortening of 

 the atrium ; (2) the tympanic chin-plate ; (3) the compressed resister ; 



(4) the hexagonal, tessellated trachelos pinched against the resister by 



(5) the lever. The position of the lever, when ojjen, is also indicated. 

 Fig. 31b, outer aspect ; a diagram representing the th}-owing off of the 

 pore and atrium, or the transition of the spiracle from the third to 

 fourth stage. (Compare the left-hand part of the diagi'am with fig. 34.) 

 Fig. 31c, diagram representing the same, but viewed from above. 



Fig. 32. — Spiracle III (the fir.st abdominal) of T. natalensis, 

 female. External aspect viewed from the side, illustrating a distinct 

 transformation to the bivalvuhir form in which B. the resister is 



