Fig. 396.—D, left dorsal trachea; S, left stigmatal trachea; I, II, III, first, second, and third pairs of abdominal dilated tracheæ, forming a plexus behind the ovaries; 1, pair of enormous thoracic air-sacs; 2, pair of smaller air-sacs; 3–7, abdominal air-sacs; oc, ocular dilated trachea and air-sacs; c, cephalic trachea. The relations of the heart to the dorsal tracheæ are indicated.—Drawn by Emerton from dissections by the author.
In the youngest larva of Corethra plumicornis Weismann ascertained the thickness of the longitudinal stem to be 0.0017 mm. That of the finest tracheal endings in the silk-glands of the silkworm was found by Von Wistinghausen to be 0.0016 mm. (Zeits. f. Wiss. Zool. xlix, 1890, p. 575.) Weismann states that in the larvæ of Corethra and Chironomus the tracheal system is only incompletely developed; the tracheæ are not united with each other, and in the youngest larvæ they do not contain air.
Fig. 397.—Tracheal system of the right side of Machilis maritima: k, head; I, II, III, thoracic segments; 1–10, abdominal segments; s, stigma.—After Oudemans, from Lang.
Each of the two main tracheæ, as Kolbe states, sends off into each segment of the body three branches.
1. An upper or dorsal branch, which supplies the muscles of the dorsal region.
2. A middle (visceral) branch, whose twigs pass to the digestive canal and back to the organs of reproduction.
3. A lower (ventral) branch, whose twigs are distributed to the ganglia and to the muscles of the ventral region.
In certain Thysanura, as a species of Machilis (Fig. 397), we probably have the primitive condition of the tracheal system, the longitudinal and transverse anastomoses being absent, but in other Thysanura (Japyx, Nicoletia, Lepisma, and a few species of Machilis) they are present.
As Kolbe remarks, whether the fine ends of the tracheæ are closed or open, whether after the analogy of the blood capillaries of vertebrates they anastomose with each other, whether the ends of the air-tubes pass between the cells or penetrate into them, these questions are not fully settled. According to Leydig’s[[61]] latest views the tracheæ penetrate into the cells and unite with the hyaloplasma. Hence the process of respiration in the last instance takes place in the hyaloplasma. This assumption accords with the fact that in the tracheate Arthropods the terminations of the tracheæ carry the atmospheric air into the space bounded by the cellular network, also to the hyaloplasma filling the spaces. Leydig[[62]] also thinks that the finest tracheal endings penetrate into the muscular tissue and unite with the primitive muscular fibres.