| Vanessa io | length of body | 32 mm.; of the silk glands | 26 mm. |
| Smerinthus tiliæ | length of body | 63 mm.; of the silk glands | 205 mm. |
| Bombyx mori | length of body | 56 mm.; of the silk glands | 262 mm. |
| Antheræa yamamaya | length of body | 100 mm.; of the silk glands | 625 mm. |
| Telea polyphemus | length of body | 60 mm.; of the silk glands | 450 mm. |
Thus in Telea the silk-glands are about 18.50 inches in length, being about seven times as long as the body.
For the most complete accounts of the spinning glands of Lepidoptera and their mechanism we are indebted to Helm and to Blanc, and for that of the Trichoptera to Gilson.
The unpaired portion, or spinning apparatus (filière of Lyonet), is divided into two portions; the hinder half being the “thread-press,” the anterior division the “directing tubes.” The silk material, stored up in the thickest portion of the glands, passes into the thread-press (Fig. 334, A), which is provided with muscles which force the two double ribbon-like threads through the directing tube, as wire is made by molten iron being driven through an iron plate perforated with fine holes. The entire spinning apparatus, or filator, as we may call it, is situated in the tubular spinneret. The opening of the spinneret is directed anteriorly, and the anterior end of the directing tube passes directly into this opening so that the directing tube may be regarded as an invagination of the lingua.
The silk thread which issues from the mouth of the spinneret is, as Leeuwenhoek discovered, a double ribbon-like band, as may be seen in examining the silk of any cocoon.
The process of spinning.—Since the appearance of Helm’s account, Gilson, and also Blanc, have added to our knowledge of the way in which the silk is spun and of the mechanism of the process. Gilson has arrived, in regard to the function of the press or filator, at the following conclusions: 1, the press regulates the thread, it compresses it, gives it its flattened shape; 2, it regulates the layer of gum[[52]] (grès) which surrounds the thread; 3, it may render the thread immovable by compressing it as if held by pincers.
The process of spinning in the silkworm, says Blanc, comprises all the phenomena by which the mass of silk contained in the reservoir is transformed into the silk fluid of which the cocoon is spun. The excretory canals each contain a cylindrical thread of silk having a mean diameter of 0.2 mm. and surrounded by a layer of gum (grès) which in the fresh living organ exactly fills the annular space situated between the fibroin cylinder and the wall. Arrived within the common duct, the two threads receive the secretion of Filippi’s gland, where the silken fluid is formed, but has not yet assumed its definite external characters. The two threads press through the common canal and arrive at the infundibulum (Fig. 334, c) of the press, at the bottom of which is situated the orifice of the spinning canal, almost completely divided into two by the sharp edge of the rachis (Figs. 334, a, 335, l). The threads each pass into one of the two grooves, and the layer of gum (grès) fills the rest of the canal of the press or filator.
Fig. 331.—Longitudinal section of the spinneret: a, horizontal portion of the tongue; b, vertical portion; c, f, circle of the tongue; d, tongue-pad; e, orifice of the spinneret; g, body of the lyre; h, prebasilar membrane forming a fold; i, internal canal of the spinneret; k, filator.—After Blanc.
