ANATOMY AND PHYSIOLOGY 



FIG. 105. 



two ducts join into a short common duct, which passes 

 through the tubular spinneret. Two divisions of the spinning 

 tube are distinguished : ( i ) a posterior muscular portion, or 

 thread-press and (2) an anterior directing tube. The thread- 

 press combines the two streams of 

 silk fluid into one, determines the form 

 of the silken thread and arrests the 

 emission of the thread at times, besides 

 having other functions. The silk fluid 

 hardens rapidly upon exposure to the 

 air; about fifty per cent, of the fluid 

 is actual silk substance and the re- 

 mainder consists of protoplasm and 

 gum, with traces of wax, pigment, fat 

 and resin. 



A transverse or radial section of a 

 silk gland shows a layer of glandular 

 epithelial cells, with the usual intim.a 

 and basement membrane (Fig. 105) ; 

 the cells are remarkably large and their 

 nuclei are often branched; the intima 

 is distinctly striated, from the presence 

 of pore-canals. The glands arise as 

 evaginations of the pharynx (ectoder- 

 mal) and the chitinous intima of each 

 gland is cast at each moult, along with 

 the general integument. 



The silk glands of Trichoptera are essentially like those of 

 Lepidoptera, but the glands of Chrysopa, Myrmcleon, Coc- 

 cinellidse, Chrysomelidse and Syrphiclse, which open into the 

 rectum, are morphologically quite different from those of 

 Lepidoptera. 



3. MUSCULAR SYSTEM 



The number of muscles possessed by an insect is surpris- 

 ingly large. A caterpillar, for example, has about two 

 thousand. 



Sections of silk gland of 

 the silk worm. A, radial; 

 B, transverse, b, basement 

 membrane; i, intima; s, 

 glandular cell with branched 

 nucleus. After HELM. 



