194 



INSECTS 



while the females are practically incapable of flight. 

 They pair readily in confinement, are hardy, and very 

 fecund, the female producing several hundred eggs. 

 All these advantages, together with the ease of handling 

 the caterpillars, are matters in favor of the mori, and 

 its propagation is largely the work of women and 

 children who do it as a side issue and hence very cheap- 

 ly. It is this latter factor indeed that has barred silk 

 culture in America, where the cost of labor is too high 

 to make the venture attractive to any class. 



It has already been said that the silk is in the nature 

 of a salivary secretion; but that is only partly true, 



Secret mg C/dnds 



Fig. 89.— Salivary gland of silk-worm. 



because, although the material is secreted by one pair 

 of what are usually salivary glands, these glands have 

 been so enormously enlarged that they extend along 

 the sides of the body for almost the full length of the 

 caterpillar, and the material has become so sticky and 

 viscid that it could not possibly have any digestive 

 function. The two glands unite into a single outlet 

 on the lower lip of the caterpillar, and as soon as the 

 fine thread of liquid issues from the opening of the 

 spinning organ and comes into contact with the air, 

 it hardens sufficiently to hold its form, reaching its 

 full strength and elasticity a few moments later. As 

 to the value of silk products each year, they must be 

 figured in millions of dollars, so that the humble cater- 



