122 



PRACTICAL ZOOLOGY 



be judged from the fact that it causes an annual loss of about 

 sixty million dollars to the people living in the fever district. 



The relations between the tick and Texas fever were definitely 

 established by Theobald Smith of the Bureau of Animal Industry, 

 U. S. Department of Agriculture, in 1889. The protozoan para- 

 sites occur in the blood corpuscles of sick cattle. The ticks suck 

 the blood of these cattle and of course take the parasites into 



FIG. 67. Texas-fever tick. 



A, adult female ready to lay its eggs. 



B, adult female and egg mass. (After Graybill.) 



their alimentary canals. When completely gorged with blood, 

 they drop to the ground ready to lay their eggs. The parasites 

 do not remain in the alimentary canal of the tick, but penetrate 

 into other regions, including the reproductive organs. They are 

 thus present in the eggs laid by the tick. 



Each female tick deposits about 2000 eggs on the ground 

 (Fig. 67). The young or " seed ticks," which hatch from these 

 eggs in a few weeks, are parasitized, since the eggs from which 

 they developed contained parasites. They are about ^ of an 

 inch long and have only three pairs of legs. When cattle brush 



