INSECTA. 297 



throwing off the last skin of the larva. The thread of the last- 

 named insect, the silk-worm, is of great tenacity ; and, notwith- 

 standing its fineness, may be wound off from the cocoon in a con- 

 tinuous thread, forming the important article of commerce, silk. 



(342.) Nothing can be more simple than the apparatus provided 

 in caterpillars for the production of this valuable commodity : 

 Placed on each side of the intestine are two long and tortuous se- 

 creting cseca (Jig. 134, v, x 9 y)> that separate from the surrounding 

 juices of the body a tenacious viscid fluid which is liquid silk. The 

 viscid secretion thus formed is in the silk-worm of a golden yellow 

 colour, and is conveyed by the excretory ducts of the secerning 

 organs (t>, z) to the labium or under-lip, where the ducts terminate 

 at the base of a tubular instrument, the fusulus or spinnaret^ 

 through which the silk is drawn (Jig. 136, c). The fusulus of 

 the silk-worm, represented in the Fi s- 13 6. 



annexed figure upon an enlarged 

 scale, is a simple nipple-shaped pro- 

 minence, perforated at its extre- 

 mity, and surrounded by four rudi- 

 mentary palpi. When about to 

 spin, the larva, by placing the ex- 

 tremity of its spinnaret in contact 

 with some neighbouring object, al- 

 lows a minute drop of the glutin- 

 ous secretion to exude from its ex- 

 tremity, which, of course, adheres 

 to the surface upon which it is 

 placed : the head of the silk-worm 

 being then slowly withdrawn, the 

 fluid silk is drawn out in a delicate 

 thread through the aperture of the spinnaret, its thickness being 

 regulated by the size of the orifice, and, immediately hardening 

 by the evaporation of its fluid parts, forms a filament of silk which 

 can be prolonged at the pleasure of the animal until the contents 

 of its silk reservoirs are completely exhausted. 



(343.) Such is the structure of the larva of a Lepidopterous insect, 



by its hind-legs. The skin of the caterpillar then gradually splits down the back (B, c), 

 and is slowly pushed upwards towards the tail of the chrysalis. The 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.) 



