152 SCORPIONS, SPIDERS, AND TICKS 



the crab spiders, so called because they riui sidewise like 

 a cralj; and the jiiinping spiders which catch their prey by 

 leaping iip(jn it. 



Economic importance of the Arachnida. — The silk of 

 spiders has lieen utilized to some extent in making cloth; 

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

 difficult to rear si)id(>rs in sufficient numbers to produce 

 silk in paying ffuantities. Spiders are of some benefit in 

 destroying injurious insects. Finally, threads of spider's 

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

 indispensaljle for this pui-pose. 



The chicken mite often b: -comes a serious pest to poultry 

 and undoul:)tedly causes considerable pecuniary loss by a 

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

 leaf Ijlistei' mite is ]:)arasitic upon the leaves of pear trees 

 and in some localities inflicts serious injuries on these trees. 

 Certain species of mites ai'e often very troulilesome to house 

 plants, especially in greenhouses. In California, citrus 

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

 sometimes causes sevei'e losses by A\eakening 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 thoi-ax are combined into a 

 cephalothorax; they have no antenna;; the eyes are simple, 

 and they do not possess gills but breathe air direct])', either 

 tlu'ough a system of tracheal tubes or by means of lung 

 sacs. Manj' of the arachnida are provided with glands 

 from whicli silk is spun. A few, the scoi-pioii, tarantula, 

 and one species nf s|)ider in the Uinted States, are more or 

 less poisonous. "Willi few exceptions, they are carnivorous, 

 uiidoul)tedly destroying many noxious insects. 



