Fig. 338.—A, section of gland of lepidopter: B, section of silk-gland of a saw-fly larva; n, nucleus; i.d, canals; d.s, cavity.—After Gilson.
Histologically the silk-glands are composed of three layers,—the outer, or tunica propria (Fig 337); the inner, the tunica intima; the middle layer being composed of extraordinarily large epithelial cells which can be seen with the naked eye, and are also remarkable for the branched shape of the nuclei (a, b, c, 337), the branches being more or less lobed, and the larger the cells the more numerous are the branches of the nucleus. Gilson[[53]] finds that those of Trichoptera, Lepidoptera, Diptera, and Hymenoptera ordinarily consist of a small number of cells; and it is quite common, he says, to find only two cells in a transverse section (Fig. 338, A). In the Tenthredinidæ, however, “the organ still consists of a tube, the wall of which is composed of flat cells, but in addition to that, two series of spheroidal cells are attached to the sides. Each of these cells contains a system of tiny canals running through their cytoplasm (B, i. d). These cells are the secreting elements; they continually cast the silk substance into the tube.” A peculiarity of the tunica intima is its distinct transverse striation.
Fig. 339.—Branching nucleus of spinning gland of Pieris larva.—After Korschelt, from Wilson.
Fig. 340.—Filippi’s glands (G) isolated and seen from above: e, e, its lobules; d, its excretory canal; E, silk-duct; C, common canal; c, upper spinning muscle; b, lower muscle; a, lateral muscle; T, spinneret.—After Blanc.
The lining of the glands and of their common duct is moulted when the caterpillar casts its skin, and this, as well as the mode of development, shows that the glands are invaginations of the ectoderm. Gilson finds that the silk-glands and silk-apparatus of Trichoptera are very similar to those of caterpillars, and that the silk is formed in the same way.
Appendages of the silk-gland (Filippi’s glands).—In most larvæ there is either a single or a pair of secondary glands which open into the spinning glands near their anterior end. They are outgrowths of the gland provided with peculiarly modified excretory cells or evaginations of the entire glandular epithelium. Those of Bombyx mori (Fig. 340) are very well developed, and, according to Blanc, form two whitish, lobulated masses in the labium on each side of the common duct of the spinning gland. Externally they appear to be acinose; but their structure, as described by Blanc and by Gilson, is very peculiar. Helm thinks, with Cornalia, that the function of these glands is to secrete the adhesive fluid which unites the silk threads, and also to make the silk more adhesive in the process of spinning, but Blanc states that this is done before the thread passes into the common excretory tubes, and he is inclined to think that the secretion serves to lubricate the spinneret, and thus to facilitate the passage of the thread. On the other hand, in certain caterpillars these glands are situated quite far from the spinning apparatus.
The silk-glands in the pupa state undergo a process of degeneration, and finally completely disappear. They are specific larval organs evolved in adaptation to the necessity of the insect’s being protected during its pupal life by a cocoon. (Helm.)