IMTRODUCTION. 
95 
pushing the hairs into the interstices of the silk. 
Others employ, for the same purpose, pai-ticles of 
earth, or the raspings and gnawed portions of the 
wood on which they subsist. Ifost of the species 
which retire under ground to p:iss this inactive 
period of their existence, form structures of con- 
siderahle strength, in which very little silk is em- 
ployed ; hut 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, rmited 
into a pretty compact fabric, which renders them 
impervious to air and moisture. Of these, one 
of the hiindsomest and most familiarly knoum 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 ^■is- 
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 reaiing its interior and more 
substantial stmeture. In a few cases, however, tliis 
exterior envelope is a compact tissue, closely em- 
bracing the other, so that the chrystdis appears to 
