238 
THE ART OP REARING 
Substance, which is purified and gradually circu- 
lated into the silk reservoirs, and afterwards 
spun into silken threads, with which the insects 
form the cocoon. The insect is therefore, under 
this view, only to be considered as an instrument 
for extracting the silky substance from the leaf of 
the mulberry tree. And they can only extract as 
much as that leaf will yield. 
This being the case, it might be concluded 
that all varieties of silk-worms are equally good, 
and it would be unnecessary to examine the ad- 
vantage or perhaps the loss that might arise from 
rearing any particular sort of silk-worm. 
However, as the term of life of the various 
species is not alike, and as the different worms 
afford silk of differing value, it is necessary to 
explain these differences, the result of which 
may be of some importance. 
There is besides a considerable variety in the 
quantity of x’esinous matter afforded by the leaf 
of the grafted mulberry, and that of the wild 
mulberry. To state this clearly, I shall speak : 
1st. Of the small silk-worm of three casts, or 
moultings. 
2nd. of the large silk-worm of four casts. 
3rd. Of the common white silk-worm of four 
casts. 
4th. Of the common yellowish silk-worm of 
four casts. 
