526 Wisconsin Acade^ny of Sciences, Arts, and Letters. 
value. It is usually looked upon as an objectionable visitor 
and consequently destroyed whenever encountered. 
After making counts in representative fields, it was 
found that there are approximately ten thousand Araneas on 
an acre of land. Since the average farm in Wisconsin and in 
the United States is about one hundred and sixty acres, this 
would mean that there would be about a million spiders. 
The experiments described in this paper show that a spider 
will consume an average of two and one-half fruit flies per day. 
Assuming that the fly season lasts for six months of the year, 
the spiders on an average farm will destroy approximately four 
billion flies per year. 
These figures show conclusively that the services of the spider 
are of great value, for it would be difficult to destroy such an 
enormous number of flies by artificial means. 
That the fly is a very destructive as well as dangerous pest 
is now a matter of common knowledge. The injury done by 
this insect may be summarized as follows: It carries disease, 
aids in the accumulation of filth and dirt in buildings and 
houses, decreases the milk flow among the cattle, and is generally 
annoying to the human race. 
The value, then, of the spider lies in its ability to catch and 
destroys flies, which is, indeed, a great service to society. If the 
spider were not in existence to assist materially in the exter¬ 
mination of the fly, mankind would be seriously handicapped, 
and no doubt if people in general would realize the great 
economic importance of the spider, and cooperate with it in¬ 
stead of carrying on a campaign of extermination, dangers 
from the fly plague would be greatly lessened. 
BIBLIOGRAPHY 
1. Comstock, J. H. 
1912. The Spider Book. New York, xv. 721. 
2. McCook, Henry C. 
1889. American Spiders and Their Spinning Work, 
Philadelphia. 3 volumes. 
3. Emerton, J. H. 
1883. Structure and habits of Spiders. Boston, i i i i. 
118. 
