Fig. 391.—Portion of a trachea of a caterpillar, with its branches B, C, D: a, peritracheal membrane; b, nucleus.—After Leydig, from Gegenbaur.
Fig. 392.—Structure of a trachea, diagrammatic: portions of the peritracheal membrane (hy) and chitinous intima (cc) removed to show the structure; in the chitinous intima or endotrachea (cc) can be seen the spiral thickenings or tænidia.—After Lang.
It will much simplify our conception of the nature of the air-tubes when we learn that they originate in the embryo as tubular ingrowths of the integument (ectoderm), these branching and finally reaching every part of the interior of the body. They are elastic tubes, and being filled with air are silvery in color, though at their origin near the spiracles they are reddish or violet bluish; or, in the larva of Æschna, reddish brown, this tint being due to a finely granular pigment situated in the peritoneal membrane.
Fig. 393.—Longitudinal section of the trachea of Hydrophilus piceus: ep, epithelium; cu, cuticula; f, spiral threads.—After Minot.
In their essential structure the tracheæ consist of the chitinous intima, which is a continuation of the cuticle of the integument, and of a cellular membrane or outer layer of cells (a continuation of the hypodermis) called the peritoneal membrane, or ectotrachea (Figs. 392, 393).
Leydig discovered that the spiral filaments are not distinct and separate, but intimately connected with the inner membrane (intima), and he detected the outer or peritoneal membrane, which Chun afterwards found to be epithelial in its nature, Minot stating that it is a true pavement epithelium.
Figure 393 represents a longitudinal section of a large trachea of Hydrophilus, showing the peritoneal membrane (ectotrachea, ep) and the intima or endotrachea, divided into the cuticula (cu), with the darker colored inner layer, in which are embedded the dark-colored tænidia (f).