CHAPTER XIII 

 THE RELATIONS OF ARACHNIDS TO MAN 



ARACHNIDS are indirectly of importance to man as destroyers 

 of injurious insects and because of their injuries to vegetation, 

 but principally because some of them transmit disease germs 

 from one animal to another, very much as do the house fly, mos- 

 quitoes, and certain other insects. (See Chapters VIII, IX, 

 and X.) 



Arachnids Destroy Insects. -- Spiders, harvestmen, and 

 scorpions are all carnivorous and feed principally upon insects. 

 The number of injurious insects they destroy annually can hardly 

 be estimated, but it must be very large, considering the abun- 

 dance and voraciousness of spiders. 



Spider Silk. The silk with which spiders build their nests 

 and webs is of excellent quality but difficult to obtain. It must 

 be collected from individual spiders in captivity, and each spider 

 yields only about an ounce. The silk is, nevertheless, sometimes 

 woven into cloth. More important than this, however, is the 

 use of the delicate silk threads as cross hairs in telescopes. 



Mites and Ticks. The mites and ticks are the arachnids 

 that act as parasites on man and domestic animals and sometimes 

 distribute disease germs. Those discussed in the following para- 

 graphs are the tick which causes Texas fever in cattle, the ticks 

 and mites that attack chickens, the mites that cause mange and 

 scab of domestic animals, and the spotted-fever tick, follicle mite, 

 itch mite, and chiggers that parasitize man. 



Texas-fever Tick. The Texas-fever tick transmits a pro- 

 tozoan parasite, named Piro plasma bigeminum, from sick cattle 

 to healthy cattle in the South. How serious this disease is may 



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