TICKS — BISHOPP 393 



may let go and crawl away to avoid danger. This habit is found 

 among certain of the leathery-bodied ticks, such as the fowl tick and 

 the relapsing fever tick. These ticks usually engorge at night and 

 hence remain in close proximity to their hosts. In the case of the 

 adults of the hard-backed ticks, such as the common wood ticks, little 

 engorgement takes place during the first 2 or 3 days, but toward the 

 end of the feeding period the bodies of the females distend with 

 blood very rapidly. During feeding salivary secretions are injected 

 into the host, and in some species these are very irritating. Tliis 

 secretion and the beak of the tick, which is sometimes broken off and 

 left in the skin when a long-beaked species is removed, are respon- 

 sible for the local lesions which may be inflamed and itch for weeks 

 or even months after the tick is gone. As the ticks of species of 

 Ornithodoros become distended with blood, a copious excretion of 

 clear fluid is thrown out by glands near the base of the legs. In 

 ticks infected with relapsing fever this excretion carries the disease 

 organisms, and the fluid entering the wound made by the beak may 

 be a medium of infection. 



Some kinds of ticks attach to only certain parts of animals, while 

 others are not particularly restricted in this respect. The spinose 

 ear tick is a good example of the restricted attachment habit. This 

 tick occurs deep in the external ear. The tropical horse tick, 

 Dermacentor nitens Neum., has similar habits but does attach occa- 

 sionally in the mane and elsewhere on horses. Most ticks attach to 

 various parts of an animal but prefer the dewlap, shoulders and the 

 region between the legs. 



LENGTH OF LIFE 



In general ticks are relatively very long lived. The length of life 

 without food of the different stages of ticks is of considerable econo- 

 mic importance, as it must be taken into account in efforts to destroy 

 these pests by starvation. Many species will live for upward of 8 

 months shut up in pill-boxes or tubes without food or water. This 

 is true of the seed ticks, nympths, and adults. The greatest longevity 

 recorded is in the case of the adult fowl tick which we have found 

 to live for nearly 2i/^ years with no food whatever. 



TICKS AND ROCKY MOUNTAIN SPOTTED FEVER 



Rocky Mountain spotted fever is a very serious human disease 

 w^hich is carried to man exclusively by ticks. There are probably 700 

 to 800 cases a year in this country, and the mortality in parts of Mon- 

 tana is over 70 percent. (See pi. 1.) More cases occur in Idaho 

 than any other State, but the virulence of the disease is much lower 

 there, the mortality running from only 5 to 8 percent. 



