104 
THE ART OF REARING 
As the insect grows older, at each moulting, 
its muzzle hardens, because it needs to saw and 
bite larger and older leaves. 
There appear now two curved lines opposite 
each other, upon the silk-worm’s back- 
The length of the silk-worms, in the first age, 
was rather less than four lines ; in the second 
age, of rather more than six lines. 
In four days it has increased its average weight 
fourfold ; when issuing from the first moulting, 
3240 silk- worms formed one ounce, at this period 
610 will form this weight. 
As the insect grows, it breathes more freely, 
its excrements are more plentiful, which, as the 
number of hurdles also increase in the laboratory, 
makes it necessary the interior air should be more 
renovated in the laboratory ; and to effect this, 
the ventilator in the floor, and the aperture made 
in the door, should be opened. (Fig. 18). 
Should there be no wind or cold in the atmo- 
sphere, the ventilator may be left open until the 
thermometer has lost a degree, or, indeed, two 
complete degrees. 
Then all should be closed ; the temperature 
again rises, and thus has the interior air been 
thoroughly renewed and purified. 
