SILK GROWER'S MANUAL. 165 



In England they remain thirty days ; in France, twenty- 

 one ; in Spain and Italy, eighteen or twenty ; in our 

 Middle States, about the same ; in California, twelve to 

 fourteen ; and in India, but eleven days. 



The silkworm, like other caterpillars, is a cold-blooded 

 insect ; its temperature is that of the atmosphere in 

 which it breathes. Sudden changes from cold to heat 

 are very injurious, yet it has been found that the silk- 

 worm is capable of enduring a great degree of heat. 

 I have remarked them to be very lively when my ther- 

 mometer was from eighty to one hundred, and some 

 days as high as one hundred and seven. This heat 

 ought to have been maintained as uniformly as possible, 

 yet it was impossible in my garret. Such a degree 

 they must have sometimes endured in their native 

 forests. But when they need more heat, it is at the 

 moment of making their cocoons. If at any time while 

 they are performing this most important labor they are 

 permitted to suffer from cold, they cease from their 

 labors, and remain inactive, or move but slowly, as may 

 be discovered while the cocoons are yet transparent. 

 It has been proved, on dissection of the silkworms which 

 thus suffer and become torpid through cold, that the 

 glutinous matter in their silk reservoirs had become so 

 congealed and tenacious from cold, as to resemble 

 strong tendons, which sufficiently accounted for the 

 inability of the insect to draw forth the silken filament ; 

 yet no sooner is the temperature increased, than they 

 will resume their labors with increased activity ; but 

 will again desist, if exposed again to cold. If neg- 



