AGENTS OF DISEASE 287 



after the larval dormant period attaching itself to 

 a new and healthy host. In the same way it may 

 pass the nymphal stage on an infected host, and 

 the adult stage upon a healthy one. Although 

 the Ixodinae ticks only moult at the change from 

 larva to nymph, and again from nymph to adult, 

 the Argasinae moult at these stages and also 

 several times during both the nymph and adult 

 stages. While the Ixodinae become enormously 

 distended with blood, the Argasinae do not 

 become swollen to such a degree. 



The body of a Tick is more or less oval or 

 shield-shaped, and is covered with a very tough 

 skin, while in the adult stage four pairs of clawed 

 legs are present. Although many species have 

 eyes, others are destitute of visual organs. 

 The somewhat complex mouth-parts consist 

 generally of a paired anchoring organ called the 

 rostrum, covered with recurved hooks, and form- 

 ing the equivalent to the pedipalps of other 

 Arachnids ; and a pair of sharp mandibles which 

 work in two longitudinal channels situated on 

 the rostrum. The Tick buries its rostrum into 

 the skin of its host and anchors itself securely by 

 means of the recurved hooks, and then proceeds 

 to gorge itself upon the blood of its host. So 

 tenaciously do the Ticks hang on, that it is 

 practically impossible to pull them off without 

 leaving the rostrum behind in the wound. The 

 only way to make the Tick release its hold being 

 to cover its body with a drop of paraffin oil, 

 turpentine, or benzole. A curious microscopic 

 organism called Spirochaeta duttoni is the cause 

 of African tick fever, and the disease, as its 



