Fig. 412.—Longitudinal section of a trachea, showing the origin of the tænidia.
Fig. 413.—Origin of the tænidia from nuclei.
Fig. 412 represents a longitudinal section through a secondary tracheal branch, showing the origin of the chitinous bands, or tænidia. At t′ are pieces of six tænidia which have been moulted; ectr indicates the nuclei forming the outer cellular layer, the ectotrachea or peritoneal membrane. These nuclei send long slender prolongations around the inside of the peritoneal membrane; these prolongations, as may be seen by the figure, become the tænidia. The tænidia, being closely approximate, grow together more or less, and a thin endotracheal membrane is thus produced, of which the tænidia are the thickened band-like portions. The endotracheal membrane is thus derived from the ectotrachea, or primitive tracheal membrane, and the so-called “spiral thread” is formed by thickenings of the nuclei composing the secondary layer of nuclei, and which become filled with the chitin secreted by these elongated nuclei. The middle portion of the tænidia, immediately after the moult, is clear and transparent, with obscure minute granules, while the nuclear base of the cell is filled as usual with abundant granules, and contains a distinct nucleolus.
Fig. 414.—Tænidia and internal hairs of Zaitha.—After Stokes.
The origin of the tænidia is also well shown by Fig. 413, which is likewise a longitudinal section of a trachea at the point of origin of a branch. The peritracheal membrane or ectotrachea (ectr) is composed of large granulated nuclei; and within are the more transparent endotracheal cells; at t′ are fragments of the moulted tænidia. The new tænidia are in process of development at t; at base they are seen to be granulated nuclei, with often a distinct nucleolus, each sending a long, slender, transparent, pointed process along the inside of the trachea. These unite to form the chitinous bands or spiral threads.
Internal hair-like bodies.—In the large tracheæ of Lampyris very fine chitinous bristles project free into the cavity of the tube (Gerstaecher), while according to Leydig there are similar chitinous points in the tracheæ of the Carabid beetle Procrustes. Dugardin had previously (1849) called attention to such hairs, giving a list of the insects in which he observed them. Emery figures a section of the tracheæ of Luciola, “in wendig behaart.”[[69]] Stokes has described those of Zaitha fluminea (Fig. 414) as “internal chitinous, hair-like bodies arising from the fold of the tænidia and projecting into the lumen of the tubes.” They are hollow, their minute cavity distinctly communicating with that of the tænidium, from which they arise by an enlarged base. They end in an exceedingly fine point which is occasionally bifid or trifid. In Fig. 414, 4, several are shown attached to the wrinkles of the tracheæ near a spiracle, and at 5 is represented a transverse section of a trachea with three hairs projecting into its cavity.[[70]]
Stokes has also described “certain minute, elliptical bodies in the tænidia, each with an internal, presumably glandular, appendage, to all appearance forming part of the tænidium from which it springs.” These are shown in Fig. 414, at 1, 3, and, more in detail, at 6; those at 7, whose thickness is about 1
8000 of an inch, appear as collections of exceedingly minute, rounded apertures in a cushion-like mass. Although not commonly occurring on the tracheal membrane between the tænidia, they may be found there, as at 4.