CHAPTER XI 



Economic and Medical Importance 



ECONOMIC IMPORTANCE 



O PIDERS 



ARE AMONG THE DOM- 



inant predators of any terrestrial community. When the fauna of 

 the soil and its plant cover is analyzed, they come to light in vast 

 numbers, in such convincing abundance that it is evident they play 

 a significant part in the life of every habitat. Working in the ex- 

 ceptionally rich forest of Barro Colorado Island, Canal Zone, Eliot 

 C. Williams estimated about 264,000 spiders per acre of the forest 

 floor in a total fauna of 40,000,000 animals (nearly half of which 

 consisted of ants and mites). While the fauna of the temperate zone 

 has fewer species than the tropics, comparable habitats probably 

 support a nearly equal numerical population. In 1907, W. L. 

 McAtee found approximately 1 1 ,000 spiders per acre in woodland 

 and 64,000 per acre in a meadow near Washington, D. C. From 

 data given by Lucile Rice on animal fauna on the herbs and shrubs 

 of woodland in Illinois in May, 1934, I have estimated 14,000 spi- 

 ders per acre, a number which would be considerably swelled by 

 addition of the floor fauna. Yet substantial as these figures are, 

 they are completely outdistanced by the total found in England 

 by Bristowe in August, 1938; he calculated that 2,265,000 were then 

 present on a single acre in an undisturbed grassy area. Further- 

 more, Bristowe believes the average number of spiders per acre in 

 all England and Wales is no less, and probably much more, than 

 50,000, and that the total spider fauna is not less than 2,200,000,- 

 000,000. Even when based on these conservative figures, the spider 

 population of the United States would amount to an astronomical 

 number. 



The over-all effect of such a large fauna of predators must be 

 a very significant one. Unfortunately, it is not possible to gauge 

 accurately the importance of spiders in their environment, because 



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