346 TEXT-BOOK OF ENTOMOLOGY 



tory cells or evaginations of the entire glandular epithelium. Those 

 of Bombyx mori (Fig. 340) are veiy 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 adapta- 

 tion to the necessity of the insect's being protected during its pupal life by a 

 cocoon. (Helm.) 



Morphologically the silk-glands are by Lang regarded as modified coxal glands, 

 and homologues of the setiparous parapodial glands of chaetopod 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 larvae 

 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" 1 

 (Fig. 307, e). The larvse of the Mycetophilidse have spinning glands at the hinder 

 end of the body, as also the imago of the female of the tineid moth Euplocamus. 

 (Kennel.) The larvse 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 larvae of 

 butterflies, of certain flies (Syrphus), and beetles (Coccinellidae and some Chry- 

 somelidse) attach themselves by silk spun from the anus, so that the pupa is sus- 

 pended 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.(i(i 



Protoplasm 24.43 



Wax 1.39 



Coloring matter ........ 0.05 



Fat and resin 0. 10 



1 See also Giard, Bull. Soc. Ent. France, p. viii, 1894. 



