THE RESPIRATORY ORGANS. 49 



In the smaller tracheae the intima is apparently structureless, 

 and these end in capillary vessels of from 2*^ to 3'^ in diameter 

 in the interior of stellate or fusiform mesoblast cells (Fig. 11,5). 

 The development of the finer tracheae in the interior of cells 

 attached to the external coat of the larger vessels was first 

 observed by Weismann [2, p. 220]. It will be seen that there is 

 a striking similarity between the manner in which the tracheal 

 capillaries of insects and the blood capillaries of vertebrates 

 are developed by intracellular vascularisation. 



Whether the cuticular intima is continued into the smallest 

 tracheal vessels is doubtful, and the branching corpuscles in 

 which these terminate have been, I believe, frequently de- 

 scribed as plexuses of ganglion cells. They are not readily 

 distinguished when they do not contain air, and the air is 

 rapidly absorbed from them after death by the blood of the 

 insect. They are more easily stained than nerve corpuscles 

 by aniline dyes, and retain the colour longer when washed in 

 water or dilute spirit. Wistinghausen (/.c.) speaks of these 

 stellate cells as a terminal capillary network (Endnctz), and 

 regards all the cells as vessels. 



The Posterior Spiracles of the adult larva are situated in a pair 

 of nearly round chitinized plates of a dark-yellow colour. Each 

 presents three oblique transverse slits, partially closed by a fine 

 chitinous grating. A vertical section through the stigmatic plate 

 at right angles to the slits is represented in Fig. 11, / and 2. 



It will be seen that each stigmatic plate is surrounded by a 

 ring of chitin, the peritreme (Fig. 11, pi), which involves 

 nearly the whole thickness of the external cuticle. The inner 

 surface of the cuticular epidermis is covered with a cuticular 

 network of fine fibres, extending across the interior of the 

 slits and forming the grating (Fig. 11, 2). 



Immediately within the spiracle there is a distinct cavity {v), 

 lined by a similar cuticular network {pr) ; I shall term this 

 cavity the vestibule of the trachea. Its intima resembles the 

 external cuticle, and is quite unlike the proper intima of the 

 tracheal vessels which open into it. 



The tracheae are usually regarded as derived from a tubular 



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