THE TR AC HEAL CAPILLARIES 



437 



terminal branches pass into this network, which he calls the tracheal capillary 

 end-network (Figs. 398, 400). This last varies in thickness and spreads out under 

 the niembrana propria of the glandular mass over the entire surface of the large 

 gland-cells and on a level with the tracheal capillaries. The tracheal endings do 

 not penetrate into the cells, but are separated from the plasma of the cells by a 

 thin membrane. The traeheal capillary end-network appears as a system of line 

 tubes like the tracheal capillaries, consisting of a peritoneal layer and achitinous 

 intima (Fig. 400). The walls of these tubes are homogeneous, not porous, 

 though readily permeable by the parenchymatous fluid. The interchange of 

 gases consequently may go on easier and more vigorously in a system of richly 

 anas'tomosing tubules of the net-like mass of traeheal capillaries, than in tubes 

 ending blindly. 



While the diameter of the tracheal capillaries is 0.0016 mm. or 1 /*, that of the 

 tubules composing the tracheal capillary end-network is scarcely measurable, 

 but is less than 1 



V 



Fio. 400. Tracheal end-cells of Lampyris splendidula : tr, trachea with tcenidia ; tre, 

 tracheal capillaries. After Wielowiejski. 



These tracheal capillaries also occur on the seminal and other sexual tubes, on 

 the intestine, on the urinary tubes, on the fat-bodies, but are most easily detected 

 on the silk-glands. 



The latest researches are those of E. Holmgren, who has studied the branch- 

 ing of the tracheae in the spinning-glands of caterpillars. He prefers to call the 

 end-cells "transition cells," as they lead from the tracheal tubes proper to the 

 capillary network. This latter is formed by slender nucleated cells, often with 

 an intracellular lumen, and, according to the author, probably constituting a res- 

 piratory epithelium. He finds that both large and small tracheae may penetrate 

 the gland-cells. (Anat. Anzeiger, xi, 1895, pp. 340-G, 3 figs.; Jour. Koy. Micr. 

 Soc., 1896, p. 182.) 



b. The spiracles or stigmata 



The spiracles are segmentally arranged openings in the sides of 

 the thorax and abdomen, through which the air passes into the air- 



