CLASS ARACHNIDA 



301 



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 



B 



Fig. 197. — Mites. A, an itch mite, Sarcoptes scabiei DeGeer. Male from above. 

 Greatly enlarged. B, Texas cattle iever tick, .\fargaropus annulatus {?>iiy). Male. X 16. 

 C, chigger, Trombicula irritans (Riley). Larva. X 75. D, adult of ohigger. Greatly 

 magnified. (.4, C, and D from Ewing. " Manual of External Parasites," by permission of 

 the publisher: Charles C. Thomas; C after Oudemans; B from. Salmon and Stiles, "Cattle Ticks 

 of the United States," Bur. An. Indus., U. S. Dept. Agr.) 



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 

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

 reduced by preventive measures. 



