SPINNING CATERPILLARS. 311 



Labium, or Imvtr lip of Cassus.—a, silk-tube. 



are like a pair of distaffs for spinning the gum into a 

 silken thread. She fixes the first drop of°gum that 

 issues where she pleases, and then draws back her 

 head, or lets herself fall, while the gum, continuing 

 to flow, is drawn out and lengthened into a double 

 stream. Upon being exposed to the air, it imme- 

 diately loses its fluidity, becomes dry, and acquires 

 consistence and strength. She is never deceived in 

 adjusting the dimensions of the [united] apertures, 

 or in calculating the proper thickness of the thread' 

 but invariably makes the strength of it proportion- 

 able to the weight of her body. 



" It would be a very curious thing to know how 

 the gum which composes the silk is separated and 

 drawn off from the other juices that nourish the 

 animal. It must be accomplished like the secretions 

 formed by glands in the human body. I am there- 

 fore persuaded that the gum-bags of the silk-worm 

 are furnished with a set of minute glands, which, 

 being impregnated with gum, afford a free passage 

 to all the juices of the mulberry-leaf corresponding 



