Kupffer is likewise of the opinion that the fine tracheæ penetrate into the cells, and Lidth de Jeude asserts that they enter the epithelial cells, “each cell containing several branches.” Kölliker, Emery, etc., maintain, however, that the tracheal endings lie between the cells. Wielowiejski,[[63]] in describing the line tracheæ of the phosphorescent organs, thinks that the tracheal endings (tracheal capillaries) rarely end blindly, but anastomose with one another, forming an irregular network. The latest observer, Gilson (1893), asserts that tracheal twigs penetrate deeply into the epithelial cells of the silk glands of larval Trichoptera as well as of caterpillars, passing through their protoplasm.
Fig. 398.—Tracheal network of the male glands of Lampyris splendidula: tec, tracheal end-cells; cap, tracheal capillaries; at a, an expanded matrix.—After Wielowiejski.
Fig. 399.—Tracheal capillary end-network (tr. c. n.) of silk glands of Ocneria dispar: p, peritoneal (peritracheal) membrane.—After Wistinghausen.
A late investigator, C. von Wistinghausen, finds in the tracheæ of the spinning-glands of caterpillars a completely formed network between the terminal branches of two or several tracheal groups. The tracheal tubes of this series of 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 membrana 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 tracheal capillary end-network appears as a system of fine tubes like the tracheal capillaries, consisting of a peritoneal layer and a chitinous 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 anastomosing tubules of the net-like mass of tracheal capillaries, than in tubes ending blindly.
While the diameter of the tracheal capillaries is 0.0016 mm. or 1 µ, that of the tubules composing the tracheal capillary end-network is scarcely measurable, but is less than 1 µ.
Fig. 400.—Tracheal end-cells of Lampyris splendidula: tr, trachea with tænidia; 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.