Morphologically the silk-glands are by Lang regarded as modified coxal glands, and homologues of the setiparous parapodial glands of chætopod worms, the coxal glands of Peripatus, and the spinning glands of spiders.
In Scolopendrella, spinning glands are situated in the two last segments of the body, opening out at the end of the cercopods (Fig. 15, s.gl), and the larvæ of the true Neuroptera (Chrysopa, Myrmeleon, etc.) which spin cocoons, have spinning glands opening into the rectum. The silk forming the cocoon of the ant-lion, as Siebold and the older observers have stated, is secreted by the walls of the rectal or anal sac. Siebold (Anatomy of the Invertebrates, p. 445) states that in the larva of Myrmeleon, the silk-apparatus is very remarkable, “for the rectum itself is changed into a large sac and secretes this substance which escapes through an articulated spinneret projecting from the opening of the anus”[[54]] (Fig. 307, e). The larvæ of the Mycetophilidæ have spinning glands at the hinder end of the body, as also the imago of the female of the tineid moth Euplocamus. (Kennel.) The larvæ of ichneumons, wasps, bees, of Cecidomyia, and other Diptera, spin silken cocoons, but their glands have not yet been examined.
It should also be observed that during the process of pupation the larvæ of butterflies, of certain flies (Syrphus), and beetles (Coccinellidæ and some Chrysomelidæ) attach themselves by silk spun from the anus, so that the pupa is suspended by its tail; such glands are probably homogenetic with the coxal glands.
The silk in its fluid or soft state is mucilaginous, and according to Mulder, in the silkworm consists of the following substances, varying somewhat in their relative proportions by weight:
| Silk-fibre material | 53.67 |
| Glue (Leim) | 20.66 |
| Protoplasm | 24.43 |
| Wax | 1.39 |
| Coloring matter | 0.05 |
| Fat and resin | 0.10 |
LITERATURE ON THE SPINNING GLANDS
Helm, E. Anatomische und histiologische Darstellung der Spinndrüsen der Schmetterlingsraupen. (Zeitschr. f. wissens. Zool., xxvi, 1876, pp. 434–469, 2 Taf.)
Lidth de Jeude, Th. W. van. Zur Anatomie und Physiologie der Spinndrüsen der Seidenraupe. (Zool. Anzeiger, 1878, pp. 100–102.)
Engelmann, W. Zur Anatomie und Physiologie der Spinndrüsen der Seidenraupe. (Onderz. Phys. Lab. Utrecht, iii, 1880, pp. 115–119.)
Joseph, G. Vorläufige Mitteilung über Innervation und Entwickelung der Spinnorgane bei Insekten. (Zool. Anzeiger, 1880, pp. 326–328.)