SILK-WORMS AND SERICULTURE. 



663 



we see come out a thread of silk. This is at first cast out in any di- 

 rection, and forms a collection of cords destined to fix the cocoon that 

 is to be spun. Soon the work becomes regular, and the form of the 

 cocoon is outlined. For some hours we can see the worker performing 

 his task across the transparent gauze with which he surrounds him- 

 self. By little and little, this gauze thickens, and grows opaque and 

 firm; finally it becomes a cocoon like these I place before you. At 

 the end of about 72 hours the work is done. 



Once it has given out its first bit of silk, a worm in good health 

 never stops, and the thread continues without interruption from one 

 end to the other. You see that the cocoon is in reality a ball wound 

 from the outside inward. The thread which forms this ball is 11 miles 

 in length ; its thickness is only ^^ of an inch. It is so light that 28 

 miles of it weigh only 15^ grains. So that 2| lbs. of silk is more 

 than 2,700 miles long. 



Let me insist a moment on the prodigious activity of the silk-worm 

 while weaving his cocoon. To dispose of its silk when spinning, it 

 moves its head in all directions, and each movement is about one-sixth 

 of an inch. As we know the length of the thread, we can calculate 

 how many movements are made in disposing of the silk in 72 hours. 

 We find in this way that a silk-worm makes nearly 300,000 motions 

 in 24 hours, or 4,166 an hour, or 69 per minute. You see that our in- 

 sect yields not in activity to any weaver ; but we must add that it is 

 beaten by the marvellous machines that the industry of our day has 

 produced. 



Fig. 10. 



Fig. 11. 



Spherical Cocoon op Bombyx Mori. 



Cocoon drawn in toward the Middle. 



All cocoons are not alike. There exist, in fact, different races of 

 silk-worms, as we have different races of dogs. These differences are 

 less obvious in the animals themselves ; they are best seen in the co- 

 coons, which may be either white, yellow, green, or gray ; some are 

 round, others oval or depressed in the middle (Figs. 10 and 11). 

 The silk of one is very fine and very strong, that of others is coarse 

 and easily broken. Hence their very different values. 



All I have said applies to the silk-worm properly so called to the 

 silk-worm which feeds on the leaves of the mulberry-tree, the Bombyx 

 morl of naturalists. But, some years since, there were introduced 

 into France new species of caterpillars that produce cocoons, and 



