452 BIOLOGY AND HUMAN LIFE 



The wax obtained from bees is of great practical value, but 

 it is coming to be replaced more and more by paraffin, which is 

 obtained from petroleum. An insect product of growing impor- 

 tance, and one for which no satisfactory substitute has yet been 

 found, is lac. Lac is used as a dressing for wood and other 

 materials, as shellac, as a stiffening for felt in the making of 

 hats, as an insulating varnish in electrical work, in the making 

 of lithographer's ink and of sealing wax, and, in increasing 

 quantities, in the manufacture of phonograph records. The 

 cochineal, another member of the scale-insect family, furnishes 

 a beautiful red dye ; this was formerly used in large quantities, 

 but, as we have seen, the anilin dyes are likely to replace 

 substantially all dyes of plant or animal origin. 



The whole silk industry rests upon the fiber obtained from 

 the cocoon covering of the silk moth. Although the chemists 

 have devised ingenious processes for making artificial silk out 

 of cellulose, we shall probably continue to cultivate the silk 

 moth for a long time to come. 



Many of the beetles may be considered as useful, since they 

 destroy large quantities of dead animal remains, as do also many 

 of the ants. Thus they may be looked upon as scavengers; 

 and a few of them, in the course of their predacious activities, 

 devour forms that happen to be injurious to us, as the ladybug, 

 which eats plant lice, or the Calosoma beetle, which keeps the 

 gypsy moth in check. 



We make direct use of very few insects. Many species are 

 of indirect value as important links in that chain of life extend- 

 ing from decomposing organic matter at one end to the large 

 useful animals, and human bodies, at the other. Many species 

 of insects are essential to the propagation of various species of 

 plants, since they are the sole agents in the distribution of pollen. 



334. Mollusks. The three classes of mollusks (see page 84) 

 are rather distinct. Many species among the gastropods, as well 

 as among the pelecypods, are used for food in all parts of the 

 world, both fresh-water and marine forms being available. Our 

 most common molluscan food species are various kinds of clams, 



