308 METAZOAN PHYLA 



\\athout external signs of metamerism and without division into cephalo- 

 thorax and abdomen. Mites breathe through tracheal tubes. The 

 cephalothoracic appendages are similar to those of spiders, although the 

 pedipalpi are not so large. The abdomen shows neither the slits into 

 the air sacs nor the spinnerets at the posterior end. Nevertheless, some 

 mites can spin silken threads from openings on the ventral side of the 

 abdomen, in some cases near the anus. 



Among mites are many which are of decided economic importance. 

 The ticks are parasitic upon various animals and, since they pass from 

 one individual to another, are capable of transmitting diseases. In the 

 case of the cattle tick, which carries the organism causing Texas cattle 

 fever, the parasite is taken up by a parent mite and introduced into 

 another host by the bite of the young of that mite, transmission thus 

 involving two generations. This disease is said at one time to have 



Fig. 211.— a pseudoscorpion. (From Comstock, "The Spider Book"; copyright, 1912, by 

 the courtesy of Doubleday, Doran & Company, Inc.) X about 9. 



caused an annual loss of $100,000,000, but its frequency has now been 

 reduced by preventive measures. 



Other parasitic mites are the itch mites; the follicle mites, which live 

 in the sweat glands and hair folhcles of man and some domestic animals; 

 and the scab mites, which produce scabies. The animals generally known 

 in this country as chiggers are the larvae of red velvet mites. 



Mites that infest plants include the "red spider," which attacks house 

 plants; many that produce galls; and some that cause diseases of leaves, 

 such as the one which causes the pear-leaf blister. Still other mites are 

 found in food products, such as cheese, sugar, and preserved meats. 



332. Other Arachnids.— Among other arachnids are the well known 

 harvestmen, or daddy longlegs, which are hke small-bodied and very 

 long-legged spiders, but which have no constriction between the thorax 

 and the metameric abdomen. 



Another type is represented by the pseudoscorpions (Fig. 211). 

 These have pedipalpi much like those of the scorpions but are very much 



