708 Mr. T. H. Taylor on 



away from it. It is only on the upper wall of the gill-stem 

 that the air-containing space is well developed. On the 

 other surfaces the thin superficial layer becomes closely 

 adherent to the deep layer, and the wall is rendered com- 

 pact. As we approach the base of the gill-stem the 

 ctmtained air-space enlarges and the fibrillye lengthen. 

 This thickcDing of the dorsal wall of the gill-stem takes 

 place at the expense of the internal cavity, whicli is con- 

 sequently much reduced and perhaps quite obliterated. 

 At the place of insertion of the gill the fibrillae become 

 fewer and farther apart, and finally disappear altogether. 

 The air-containing space is now practically a tube excavated 

 in the thickness of the cuticle of the dorsal wall of the gill. 

 The air-tube now leaves the gill, and enters the cavity 

 of the thorax, in which it lies free. After a short course 

 it ends abruptly by opening into a tracheal trunk (fig. 4, 

 x). This tracheal trunk is the tracheal extension already 

 mentioned. By means of this air-tube there is direct 

 continuity between the air in the gill and the air in the 

 tracheal system. Each gill has its own tracheal extension, 

 by which it communicates with the mesothoracic spiracle- 

 chamber of its own side. The tracheal extension (fig. 

 4, ex) is a wide tube lying near the wall of the thorax. 

 The communication between the tracheal extension and 

 the air-tube from the gill is not easily demonstrated, but 

 its existence may be inferred from the following experi- 

 ment. If a living pupa with entire gills be placed in 

 strong alcohol, the air is rapidly driven out of the filaments, 

 which consequently lose their silvery appearance. On 

 replacing the alcohol with water, air soon reappears. The 

 only possible source for the air is the tracheal extension. 

 If the air be first removed from the tracheal extension, 

 the filaments permanently lose their silvery appearance 

 after the air has once been driven out by alcohol. 



At the posterior end the extension is connected with the 

 spiracle-chamber. At this point the chitinous intima loses 

 its retiform thickening, and is developed into fibril hu, 

 which are similar to those that traverse the air-containing 

 space in the gill. A perforated diaphragm is thus formed, 

 which reduces the opening of the tracheal extension to a 

 series of extremely minute apertures. No account of the 

 meaning of this structure can be given. 



From the preceding description it will be seen that 

 the cavity of the gill is shut off from the cavity of 



