152 SCORPIONS, SPIDEES, AND TICKS 



the crab spiders, so called because they run sidewise like 

 a crab; and the jumping spiders which catch their prey by 

 leaping upon it. 



Economic importance of the Arachnida. — The silk of 

 spiders has been utilized to some extent in making cloth; 

 but it is difficult to wind the silk upon a reel, and it is more 

 difficult to rear spiders in sufficient numbers to produce 

 silk in paying quantities. Spiders are of some benefit in 

 destroying injurious insects. Finally, threads of spider's 

 silk are used to form the cross hairs in telescopes and are 

 indispensable for this purpose. 



The chicken mite often becomes a serious pest to poultry 

 and undoubtedly causes considerable pecuniary loss by a 

 decrease in the production of eggs and chickens. The pear- 

 leaf blister mite is parasitic upon the leaves of pear trees 

 and in some localities inflicts serious injuries on these trees. 

 Certain species of mites are often very troublesome to house 

 plants, especially in greenhouses. In California, citrus 

 trees are subject to the attacks of a species of mite which 

 sometimes causes severe losses by weakening the vitality 

 of the trees. We have already spoken of the injury caused 

 by the southern cattle tick. 



Chief characteristics of the Arachnida. — They have four 

 pairs of legs ; the head and thorax are combined into a 

 cephalo thorax; they have no antennge; the eyes are simple, 

 and they do not possess gills but breathe air directly, either 

 through a system of tracheal tubes or by means of lung 

 sacs. Many of the arachnida are provided with glands 

 from which silk is spun. A few, the scorpion, tarantula, 

 and one species of spider in the United States, are more or 

 less poisonous. With few exceptions, they are carnivorous, 

 undoubtedly destroying many noxious insects. 



