Fig. 405.—Diagrammatic figures of the internal apparatus which closes the trachea, in the stag-beetle: A, trachea open; in B, closed; St, the stigma, with its grated lips; Ct, cuticula of the body-walls; Vk, closing pouch; Vbü, closing bow; Vba, closing band; M, occlusor muscle.—From Judeich and Nitsche.
The closing apparatus of the stigma.—Whether the external opening of the stigma is permanently open or closed, communication with the tracheæ may be cut off at pleasure during respiration by an internal apparatus of elastic chitinous bands and rods and the occlusor muscle.
The parts concerned in this operation are: 1. The closing bow; 2. The closing lever or peg; 3. The closing band; 4. The occlusor muscle (Figs. 405, 406).
Fig. 406.—Stigma, with the closing apparatus, of Smerinthus populi (imago), seen from within: b, closing bow; c, closing band: o, stigmatic opening; r, external chitinous ring; l, closing lever; m, occlusor muscle; s, scales which lie like roofing tiles over the stigma.—After Krancher.
“The first three parts are chitinized; they form a ring around the stigmatic opening, and are united to each other by joints. The bow is usually crescentic and as a rule surrounds one-half of the trachea. On the other side is the closing band which, by different contrivances, representing the closing lever or peg, becomes closely pressed against the closing bow. This lever is usually of the shape of a slender chitinous rod, which causes the closure; but it can also bend rectangularly, become converted into a typical lever as in the Lepidoptera, or it may assume the form of two peg-like processes, which press with their base against the closing bow.” (Krancher.)
“The closure of the spiracular opening is effected by the contraction of the muscles, while the opening is due to the elasticity of the chitinous parts. When at rest the spiracle is naturally open, so that the air in the trachea can directly communicate with the external air. Usually one end of the muscle is attached to the closing peg, and the other end to the closing bow. Where, as in Melolontha, the closing apparatus is provided with two levers, then naturally the muscle binds these two together and brings about by powerful contractions a firm closure of the trachea”; but, remarks Krancher, “this is not the only kind; there are numerous modifications. Besides the form just described, the levers assume the form of valves (Sirex), or of a brush (Pulex); or of a ring (larvæ of Diptera) with a circular muscle attached to it; or of a ring which simply becomes compressed (thoracic stigmata of Diptera).”
c. Morphology and homologies of the tracheal system
As first shown by Bütschli, the tracheal system is a series of segmentally arranged tubular invaginations of the ectoderm; a pair of stigmata primitively occurring on every segment of the body except perhaps the most anterior, and the last two or last one, a reduction in their number having since taken place, until in the Podurans none have survived. In the supposed ancestor of myriopods and insects, Peripatus, there are tracheæ; but they are very fine, simple, not-branched chitinous tubes which are united into tufts at the base of a flask-shaped depression of the integument, the outer aperture of which depression is regarded as a stigma. In one species (P. edwardsii) these tufts and their openings are scattered irregularly over the body; but in another kind (P. capensis) some of the stigmata at least show traces of a serial arrangement, being disposed in longitudinal rows—two on each side, one dorsally and one ventrally, those of each row, however, being more numerous than the pairs of legs. (See p. 9 and Fig. 4, D.)