SILK-WORMS. 
5 
Some caterpillars have a smooth skin, as the 
silk-worm, others are rough, and elevated in cer- 
tain parts ; some have one either partially or en- 
tirely of velvet, covered with hair or bristles, of 
various colours, often so beautiful, lively, and so 
well shaded, that art cannot imitate them. It is 
not a part of my subject to speak of the anatomi- 
cal structure of these insects. 
2. Of the Changes which Caterpillars undergo. 
A character peculiar to caterpillars is to change 
their skin at least three times before they reach 
the period when they spin the silk which they con- 
tain, in order to assume the chrysalis state in the 
cocoon or envelope which they have formed (§ 4.) 
In the greatest number of caterpillars this 
change takes place three or four times, in others 
from five to nine times. These changes are called 
casting of the skins, and are in fact diseases that 
often destroy the lives of a great number of these 
insects (§ 6.) 
One single skin allotted to an animal which, in 
a short space, increases its weight a thousand 
times, would with difficulty have been able to 
distend itself sufficiently to cover it entirely. Thus 
provident nature has extended over the body of 
the caterpillar the embryos of the skin of each 
moulting, which supply also the hairs or bristles 
with which many kinds are abundantly covered 
