ANATOMY AND PHYSIOLOGY 85 
two ducts join into a short common duct, which passes 
through the tubular spinneret. Two divisions of the spinning 
tube are distinguished: (1) a posterior muscular portion, or 
thread-press and (2) an anterior directing tube. ‘The thread- 
press combines the two streams of 
HIG. 105. 
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 
aig -sabout fiity 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 intima 
and basement membrane (Fig. 105) ; 
the cells are remarkably large and their 
nuclei are-often branched; the intima 
is distinctly striated, from the presence 
[ re- Ole > aryc x 
of pore-canals. The glands arise as Seclinns op calese tee 
evaginations of the pharynx (ectoder- the silk worm. 4, radial; 
B, transverse. b, basement 
mal) and the chitinous intima of each 
membrane; 1%, intima; s, 
Glands cask atcach moult, alone with ' slandularcell with: busgetcd 
: 7 nucleus.—After Het. 
the general integument. 
The silk glands of Trichoptera are essentially like those of 
Lepidoptera, but the glands of Chrysopa, Myrmeleon, Coc- 
cinellidee, Chrysomelidee and Syrphidze, 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. 
