d. The spiral threads or tænidia
It is generally supposed that the so-called “spiral thread” forms a continuous thread from one end of a tracheal branch to the other. This was first shown not to be the case by Platner in 1844. Minot has proved that “there is not a single spiral thread, but several, which run parallel to one another and end after making a few turns around the trachea.”
The tænidia we have found to be in some cases separate, independent, solid rings, though when there is more than one turn the thread necessarily becomes spiral. The tænidia of a main branch stop at the origin of the smaller branches, and a new set begins at the origin of each branch. The tænidia at the origin of the branch do not pass entirely around the inside of the peritoneal membrane; in the axils they are short, separate, spindle-shaped bands (Fig. 409).
At one point in the main trachea of the larva of Datana the tænidia were seen to end singly on one side (at a considerable distance from any branch or axil) at intervals, with a tænidium situated between them, making four or five turns; then there is only one band situated between two ends; this band or thread is succeeded by a set with five turns between the two ends, this set being succeeded by one complete ring situated between two ends; in all cases the ends vary in length, some threads being short and others long, so that they apparently end anywhere along the circumference of the trachea, and this arrangement is seen to apparently extend along the whole length of the trachea. Hence it is seen that as a rule the tænidia vary much in length, and never, as generally supposed, pass continuously from one end to another of a tracheal branch, for there are many spirals in a branch, each making only from one to five turns, most usually four turns. Fig. 408, part of a trachea of Dyticus marginatus, shows that at a slight bend in a trachea the tænidia is interrupted, and short, incomplete, wedge-shaped tænidia (e) are interpolated; at A, d is seen a split in one of the tænidia (compare also MacLeod, Pl. 1, Fig. 9). The threads are quite irregular in width. In the axils of the branches there is, as seen in Fig. 409, a basketwork of independent, short, often spindle-shaped tænidia; these are succeeded by longer ones, until we have threads passing entirely around near the base of each new branch; these being succeeded by others which make from two to five spiral turns.
Fig. 408.—Tænidia of Dyticus: d, a split tænidium; e, e, ends of tænidia.
The shape of the tænidia appears to vary to a great extent. In lepidopterous insects we have observed them to be in their general shape rather flat and slightly concavo-convex, the hollow looking towards the centre of the trachea. Minot’s section (Fig. 393) shows that in Hydrophilus they are cylindrical and solid, and Chun states that those of Stratiomys are round, while in Eristalis they are round, with a ridge projecting into the cavity of the trachea; in Æschna the thread is quadrangular. MacLeod states that sometimes it is cylindrical, in other cases flat, likewise prismatic; Macloskie believes that the spiral threads of the centipede are “fine tubules, externally opening by a fissure along their course.”
Fig. 409.—Tænidia of Dyticus in an axil of two branches: e, e, ends of tænidia.
Stokes confirms Macloskie’s statements, stating that in the hemipterous Zaitha fluminea “the tænidia are fissured tubules formed within and from chitinized folds of the intima, the convexity of the folds looking towards the lumen of the tracheæ.” In Fig. 414, 1, are represented portions of several tænidia showing the fissure, which is sometimes interrupted; at 2 are seen “the formation of what may be called apertures in a chitinous bridge.” Stokes regards the tænidia as “inwardly directed folds of the membrane.” Near the spiracles the tracheal membrane is externally studded with minute papillæ, as shown at 3, where are represented three broad and incomplete tænidia, with the tapering end, or the beginning, of another. Stokes adds, “Here they are only broad grooves, with no appearance of the narrow fissure of the completed tænidium. At 4 is figured a portion of the internal surface of a large trachea near the external orifice, the tænidia being in an incipient stage, evidently forming more or less of a network, as is usually the case next to the stigma” (compare p. 451, and Fig. 414).