52 GENERAL ZOOLOGY 



The silk-glands extend through the body of the caterpil- 

 lar and open into the mouth. Toward the time of pupation 

 they increase in size and produce about four thousand yards 

 of material, which, on being exposed to the air, hardens and 

 becomes the silk of commerce. As there are two sets of these 

 glands, each silk filament is really double. Within this cocoon 

 of silk the larva casts its caterpillar skin and becomes a pupa. 

 In silk-culture the cocoons are placed in an oven to kill the 

 pupae, and the silk is softened by being placed in warm water. 

 Then, by means of a twig moved about among the cocoons, 

 loose ends of several of them are caught, united into one 

 thread, and wound on a reel, which is placed at a distance 

 from the hot water so that the silk may dry. This is the 

 raw silk of commerce. 



We have several large moths in the United States, expand- 

 ing from ten to fifteen centimeters (four to six inches), whose 

 larvae spin silken cocoons, some of which have been utilized 

 to a limited extent by man. They are magnificent insects, 

 with broad wings beautifully colored, and with large feather- 

 like antennse. The larvae feed on the leaves of forest and 

 fruit trees, and are more or less armed with a variety of 

 colored spines and tubercles. The caterpillars, like other 

 larvse which feed upon plant food, are remarkable for the 

 amount of food which they consume and the great increase 

 in size in early life. The caterpillar of the American silk- 

 worm moth (Te'lea polyphe'mus), which weighs on hatching 

 one twentieth of a grain, increases to ten times its weight 

 within ten days, and to over four thousand times its weight 

 in fifty days. By this time it has consumed over one hundred 

 oak leaves, weighing almost three quarters of a pound, and 

 drunk nearly one half an ounce of water. The larvae of these 

 moths spin conspicuous brown cocoons, which can easily be 

 collected in the winter from the branches and leaves of trees 

 to which they are attached. In the spring the imagoes escape 



