COCOON OF SILKWORM. 219 



intended purpose : it is probably a similar secretion which 

 gives to the chrysalides of the Catocalce the fine purple 

 bloom vAt\\ which they are covered in that state. Others, 

 again, as the caterpillars of the garden tiger-moths, rub 

 off the hairs from their bodies, with which they are so 

 plentifully supplied (whence they have obtained the ordinary 

 names of woolly bears), and spin them into the substance of 

 the cocoon in a very ingenious manner, so as not to permit 

 the ends of the hairs to stand upright in the inside of the 

 case, which would, of course, fret the newly-disclosed 

 chrysalis. 



The more perfectly constructed cocoons, however, such, 

 for instance, as that of the silkworm, consist apparently of 

 two distinct portions, externally a loose gauze-like covering, 

 and internally a closely woven and compact ball of an oval 

 form. As the insect works from the out to the inside of its 

 case, the former is of course first spun, but the whole is 

 formed of a single thread, disposed in two different ways ; 

 and here may be noticed the interesting circumstance, that 

 although the threads of these cocoons are fastened together 

 with gum, they are yet so slightly adherent, that they may 

 be easily unwound : if their union were more perfect it would 

 be impossible to divide the thread, which is as easily un- 

 wound as that of a ball of cotton ; but it is one of the essential 

 qualities of the gummy matter with which the thread is 

 coated, that it dries very quickly, so that no sooner is it 

 emitted from the spinnerets, than it is almost di-y, before it 

 reaches the layer of threads upon which it is about to be 

 placed. According to Malpighi, six distinct layers of silk 

 are distinguishable in the cocoon of the silkworm, but 

 Reaumur suspects there is often a greater number : be this 

 however as it may, a single thread may be unwound from it 

 more than a thousand feet long. 



Many interesting instances of the manufacture of various 

 kind of cocoons, not only composed of silk, but also of silk 



