SILK-SECERNING APPARATUS. 



355 



Fig. 179. 



se- 



with the Silkworm (fig. 175), supplying the material with which the 

 caterpillar encases itself pre- 

 paratory to throwing off the 

 last skin of the larva. The 

 thread of the last-named in- 

 sect, the Silkworm, is of great 

 tenacity ; and, notwithstand- 

 ing its fineness, may be wound 

 off from the cocoon in a con- 

 tinuous thread, forming the 

 important article of commerce, 



(921.) Nothing can he more 

 simple than the apparatus 

 provided in caterpillars for 

 the production of this valu- 

 able commodity: Placed on 

 each side of the intestine are 

 two long and tortuous 

 creting ca3ca (fig. 17 

 that separate from the sur- 

 rounding juices of the body a 

 tenacious viscid fluid, which 

 is liquid silk. The viscid se- 

 cretion thus formed is in the Silk- 

 worm of a golden-yellow colour, 

 and is conveyed by the excretory 

 ducts of the secerning organs (z) 

 to the labium or under-lip, where 

 the ducts terminate at the base of 

 a tubular instrument, the fusulus 

 or spinneret, through which the 

 silk is drawn (fig. 180, c)< The 

 fusuUu of the Silkworm, repre- 

 sented in the annexed figure upon 

 an enlarged scale, is a simple 

 nipple-shaped prominence, perfo- 

 rated at its extremity, and sur- 

 rounded by four rudimentary palpi. 

 When about to spin, the larva, by 



. . , . . . J 



placing the extremity OI its Spin- 



neret in contact with some neigh- 



^o* 1 



Process whereby a chrysalis becomes suspended by 

 the tail. (See note, p. 354.) 



Fig. 180. 



Hea <i of a Caterpillar from beneath : a a, 

 antennae; 66, horny jaws; c, thread of silk 



r itted fr ^ ^ e 5 onical ,. fu8ul s ' wh ; ch i8 



seen surrounded by four rudimentary palpi. 

 pupa now lays hold of the old skin, nipping it between the rings of the abdomen, 

 and hanging in this posture inserts the apex of the tail, which is covered with hooks for 

 the purpose, into the silk previously deposited, and thus remains fixed in safety (D). 



2A2 



