420 VFTEEINAKY HYGIENE 



sequent stages of development are passed on the animal, 

 the tick holding on by its mouth parts ; with other varieties 

 between each gorging period the tick falls to the ground, 

 undergoes a change of development, and once more gains 

 access to the host before described. 



In consequence of this habit, Theiler divides ticks into 

 three biological groups, viz., ticks with one, two, and three 

 hosts. 



The tick responsible for Eed Water passes the whole of 

 its life on the animal body, and the females drop off to lay 

 their eggs and die ; but in East Coast Fever, Heart- Water 

 of Sheep, and Malignant .laundice of the Dog, each stage is 

 passed off the animal body, further development taking 

 place in the ground, and a more perfect parasite each time 

 regaining its host. 



The number of eggs laid by the female before death 

 differs with the species, but from 4,000 to 15,000 are known 

 to be laid. We may now, with a little more detail, follow 

 the stages of development of a pathogenic tick, selecting 

 that which at present is the most interesting one, viz., the 

 ' brown tick ' of South African East Coast Fever. 



This cattle tick has three feeding stages, to explain 

 which it is necessary to begin with the eggs. These are 

 generally laid in the ground in a sheltered position, and 

 take from a few weeks to a few months to hatch out, 

 depending on the temperature. The result of their hatch- 

 ing is the extremely small larval tick, which climbs up 

 bushes or grass, and is brushed off by passing animals 

 (Fig. 175). Having reached the latter, they proceed to feed, 

 which occupies a few days, and when distended with blood 

 they drop off. The distended larval tick is at this time no 

 larger than the head of a pin. 



About a fortnight is now spent in digesting the store 

 of food accumulated. Further, a new skin has to be 

 formed to take the place of the old one, and certain 

 changes in structure occur by which the tick gains new 

 mouth parts and two more legs. This, known as the 

 nymph or second stage, is distinguished from the larval 



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