Spinning Caterpillars. 335 



deceived in adjusting the dimensions of the [united] aper- 

 tures, or in calculating the proper thickness of the thread, 

 but invariably makes the strength of it proportionable 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 therefore 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 with this glutinous 

 matter, while they exclude every fluid of a different quality."* 

 When confined in an open glass vessel, the goat-moth cater- 

 pillar will effect its escape by constructing a curious silken 

 ladder, as represented by Eoesel. 



Caterpillars, as they increase in size, cast their skins as 

 lobsters do their shells, and emerge into renewed activity 

 under an enlarged covering. Previous to this change, when 

 the skin begins to gird and pinch them, they may be observed 

 to become languid, and indifferent to their food, and at 

 length they cease to eat, and await the sloughing of their 

 skin. It is now that the faculty of spinning silk seems to 

 be of great advantage to them ; for, being rendered inactive 

 and helpless by the tightening of the old skin around their 

 expanding body, they might be swept away by the first puff 

 of wind, and made prey of by ground beetles or other 

 carnivorous prowlers. To guard against such accidents, as 

 soon as they feel that they can swallow no more food, from 

 being half choked by the old skin, they take care to secure 

 themselves from danger by moorings of silk spun upon the 

 leaf or the branch where they may be reposing. The cater- 

 pillar of the white satin-moth (Leucoma salicis, STEPHENS) in 

 this way draws together with silk one or two leaves, similar 

 to the leaf-rollers (TortricidcK), though it always feeds openly 

 without any covering. The caterpillar of the puss-moth 

 again, which, in its third skin, is large and heavy, spins a 

 Spectacle de la Nature, vol. i. 



