Fig. 332.—The lower lip (labium) of Bombyx mori, isolated, seen from the left side: A, lyre; B, spinneret; C, labial palpus; D, vertical part of the labium; E, horizontal part of the same; H, L, silk-canal; K, right gland of Filippi; L, canal of the left gland; N, labial nerve; a, oblique fibre of the elevator of the labium; b, right fibre of the same; c, depressor of the labium; d, superior spinning muscles.
Fig. 333.—The labium in a horizontal position, seen from the side: f, the filator or press situated under the external part of the spinneret (d), between the branches (b), of the lyre (a); e, labial palp; c, tongue.
Fig. 334.—Longitudinal section of the spinneret and press (filator): A, filator or press; B, spinneret; C–D, body of the lyre; F, lower part of the labium; E, common canal; eh, its epithelium; G, superior muscle of the press; a, rachis; b, its posterior enlargement; c, infundibulum; d, cuticle; o, orifice of the spinning canal; op, central canal of the lyre and of the spinneret; fi, hypodermis of the lyre; f, f, hypodermic pad of the lyre.
The silken substance is then pressed by the more or less powerful contractions of the muscles of the filator, so that the passage of the threads is facilitated. If the muscles totally contract, the spinning canal is opened wide, the threads pass easily upwards and assume the form of a triangular prism (Fig. 336).
Fig. 335.—Spinning apparatus, seen from above: A, opening of the spinneret; B, central canal of the spinneret (C); D, common canal; E, canal of Filippi; F, excretory canal of a silk-gland; i, orifice of the canal of Filippi’s gland; l, rachis; k, ring of the infundibulum; b, c, d, e, f, cavity of the different canals; h, spur which separates the two excretory canals.—This and Figs. 331–334 after Blanc.
If this contraction diminishes, the chitinous wall of the spinneret comes together, owing to its elasticity; the ceiling of the canal approaches the floor; the cavity tends to take the form of a semicircular slit, and the threads are compressed, flattened. As each mass or thread of silk is much more voluminous than the canal, except when the latter is extremely dilated, it follows that the two threads are always compressed, or squeezed together, and that each of them is compelled to mould itself in the groove it occupies and to take its shape. Hence the variations in the appearance of the two masses or divided portions of silk, which as stated present all grades between the form of an isosceles-triangular prism and that of a nearly flat ribbon; but this last case is quite rare. The use of the spinneret, then, is to compress the thread and to change its form more or less considerably, at the same time as it diminishes its diameter.
