INTRODUCTION. 95 



pushing the hairs into the interstices of the silk. 

 Others employ, for the same purpose, particles of 

 earth, or the raspings and gnawed portions of the 

 wood on which they subsist. Most of the species 

 which retire under ground to pass this inactive 

 period of their existence, form structures of con- 

 siderable strength, in which very little silk is em- 

 ployed ; but in nearly all cases the interior is lined 

 with a fine tapestry of that material, by which it is 

 rendered smooth and warm. In addition to this, 

 some use an internal layer of varnish, and others a 

 soft substance resembling paste, which they apply 

 with their head to the whole of the inside, which 

 is thus coated over something after the manner 

 of a blackbird's nest. But the cocoons most fre- 

 quently met with are composed of pure silk, united 

 into a pretty compact fabric, which renders them 

 impervious to air and moisture. Of these, one 

 of the handsomest and most familiarly known is 

 that of the silk- worm. Such cocoons are formed by 

 a single continuous thread, not wound in a circular 

 direction, but in a succession of zigzags, the vis- 

 cosity of the thread when it first issues from the 

 spinneret enabling it to adhere wherever it is ap- 

 plied. The greater number of the more closely 

 woven kinds are enclosed in a loose web, which is 

 first spun as a kind of scaffolding for the caterpillar, 

 while employed in rearing its interior an>d mora 

 substantial structure. In a few cases, however, this 

 exterior envelope is a compact tissue, closely em- 

 bracing the other, so that the chrysalis appears to 



