INSECTS AND HUMAN DISEASE 147 



adults are of such a size that they could easily be removed 

 by the smaller animals. Attachment is effected by means 

 of a spiny beak, through which blood is drawn. The male 

 feeds for a period of four days; fertilisation takes place 

 on the host, and, in a week to a fortnight, after attach- 

 ment, the female, being filled with blood, falls to the ground 

 and seeks a hiding place where she may lay her eggs. As 

 showing the great extension of which the body of the 

 female tick is capable, it is of interest to note that, prior 

 to feeding, its dimensions were one-sixth of an inch long 

 by one-tenth of an inch wide, whilst, after feeding, the 

 measurements had increased to half an inch long by one- 

 third of an inch wide, with a thickness of a quarter of an 

 inch. In fact, the distension is often so great that the head 

 and legs become very inconspicuous. The males do not 

 increase in size to nearly the same extent. During ovi- 

 position, the female shrinks in size, shrivels up and changes 

 from a bluish -grey colour to a mottled yellow, and 

 eventually dies. 



The chief hosts for the larval and nymphal ticks are 

 the Columbian ground squirrel, Citellus columbianus, the 

 pine squirrel, Sciurus h. richardsoni, and the yellow- 

 bellied chipmunk, Eutamias b. luteiventris ; and for the 

 adults, horses, cattle, mules, asses, the mountain goat, and 

 the brown bear. The adult is the only stage that noi mally 

 attacks man. The mountain goat is probably the natural 

 reservoir of the virus of Rocky Mountain spotted fever, 

 and the immature stages of the tick are probably carried 

 by ground squirrels from the higher altitudes, where the 

 mountain goats feed, to the lower levels, where the cattle 

 graze. 



In addition to the Rocky Mountain spotted fever, the 

 New World is responsible for another disease of mankind, 

 probably carried by ticks, and known as verruga fever; 

 to be perfectly honest, ticks have not been actually proved 

 to be the vectors, but many things point to the probability 



