SILK-WORMS. 
169 
which demonstrates that from 2190 pounds, a part 
served to nourish the silk-worms, and the rest ex- 
haled in vapour. 
On comparing the dung produced in this age 
with the quantity of leaves given to the silk- 
worms, it will be found to be in greater proportion 
than that produced in the other ages. We shall 
explain the reason of this. (Chap. VIII. § 4.) 
Calculating the weight of the leaves, and the 
loss by evaporation of the moisture, of which we 
shall speak presently (Chap. XIV.), the worms 
must have consumed, in their fifth age alone, 
1200 pounds of leaves per ounce of eggs. 
We shall see, in the following Chapter, that the 
worms accomplish the fifth age, and cast their 
skin in their transition to the chrysalis, when they 
have lost more than half their weight. 
We then see those marks on the backs of the 
silk-worms which I have already mentioned ; and 
the shelly proboscis, black and shining, which is at- 
tached to the muzzle, acquires a very considerable 
force. At this period of progress the worm is 
whiter than ever. 
As I must suppose these insects to have been 
reared upon the system I have described, they 
will have been uninterruptedly sleek and healthy. 
I have already said, that should it be impos- 
sible, from the heat of the season* to keep the 
i 
