106 ECONOMIC ZOOLOGY 



Probably all the conditions for the production of raw silk could 

 be as well met in this country as anywhere else except that of handling 

 the cocoons; so long, as this must be done by hand, it is not likely that 

 the industry here can successfully compete with the cheap Oriental 

 labor, as noted above. 



Among the beneficial insects, perhaps the most important are 

 those that help us control the destructive ones. These may be divided 

 into the predacious forms that attack and devour other insects, and 

 the parasitic forms that spend a part of or all their lives on or in the 

 bodies of other insects. Among the predacious forms the ladybird- 

 beetles are perhaps the most important. These little beetles destroy 

 enormous numbers of scale-insects, plant-lice, etc. One of the best 

 examples of their work is seen in the work of the Australian ladybird- 

 beetle, Nowus cardinalis, that was introduced, some years ago, into 

 California to control the cottony-cushion scale that threatened to de- 

 stroy the citrus fruit industry in that State. The beetles soon had the 

 scale under control and it is now no longer considered a serious pest. 

 Other species of ladybird-beetles have been successfully introduced 

 into the United States and many of our native species are destructive 

 to certain species of aphis and scale-insects. 



Among the parasitic insects are many that like the ichneumon 

 flies, lay their eggs upon or in the bodies of caterpillars; when the 

 eggs hatch the larvae feed upon and eventually kill the caterpillar, 

 but not until the parasites have reached the pupa stage, ready to trans- 

 form into the adult insects which repeat the process, Fig. 79. But for 

 the work of these predacious and parasitic insects the control of the 

 destructive forms would be a much more serious problem than it 

 already is. 



The value of dragon flies in the destruction of mosquitoes has 

 alreadv been mentioned. 



