THE ECONOMIC BIOLOGY OF INVERTEBRATES 473 



America and Africa; their abdomen ends in a venomous sting, 

 which, while painful, is seldom fatal to man. 



" Daddy-long-legs," which belongs to this group, is a very use- 

 ful citizen because he feeds almost entirely on plant lice. 



Mites and ticks are degenerate parasitic forms which live on 

 the blood of mammals such as the dog, cattle, and man. The 

 itch is a disease produced by a mite, but, 

 thanks to the popularity of soap, it causes 

 little trouble. 



Insects. The economic relations of in- 

 sects are so important and complicated 

 that we can only summarize them here. 

 Refer to any of the books on " economic 

 zoology " to get a full idea of their im- 

 portance. Over half of all insects are 

 harmful, 250 species attack the apple, 

 grape, and orange, alone. 



As to their harmful activities, they 



1. Destroy grain, vegetables, and fruit 

 crops. 



2. Convey many kinds of disease (flies 

 and mosquitoes). 



3. Injure domestic animals (flies and 

 mosquitoes, etc.). 



4. Destroy buildings, clothing, etc. 

 (white ants and " moths "). 



5. Annoy and injure man by bites and stings. 



Their total damage in United States is over $200,000,000 per 

 year. 



On the other hand, we owe to the insects many useful processes 

 and products such as 



1. Pollenation of flowers of valuable plants. 



2. Acting as scavengers (maggots, beetles). 



3. Killing injurious insects (lady bugs which eat scale insects 

 and ichneumon flies that destroy tree borers). 



4. Furnish silk (silk moth cocoon). 



FIG. 152. A scorpion, 

 Centrums sp., from Cali- 

 fornia. (Natural size.) From 

 Kellogg. 



