482 INFLUENCE OF CLI-MATE uN ANIMAL DISEASE. 



in the same way, ma}- engorge and moult to the nymphal stage 

 on the host. The nymph then engorged drops to the ground to 

 moult to the adult stage, when a new host has again to be foiuid. 

 Species with a life-cycle of this kind are included within the 

 " two-host '" ticks, and here an example is R. cvertsi. Another 

 condition of affairs still exists in those cases where the tick drops 

 from the host, first between the larval and nymphal stages, and 

 then again between the nymphal and adult stages. These 

 ticks have thus to find hosts first as larvje, then as nympha% 

 finally as adults, and hence are known as " three-host " ticks. 

 An example of this kind is R. appendicidatus. 



To learn now something of the eft'ects of climatic and 

 tellurical factors on the development of some of these species in 

 South Africa, we shall take as exam])les the blue tick Rhipiccpha- 

 liis dccoloraius), the brown tick { RJiipiceplialiis appendicidatus), 

 and the bont tick (Ainhlyonima Hcbracitui). 



The first of these is perhaps the^most common South Afri- 

 can species, and is very widespread in distribution. Iniller, 

 speaking of it in the Cape Province, says tliat it is the most 

 M'idely-distribtited tick in that Province, and Theiler, speaking 

 of the, Transvaal, says that it occurs everywhere, but is certainly 

 less frequent on the high than on the middle and low veld. It 

 is known to be responsible for the transmission to cattle of the 

 protozoan parasites causing redwater. anaplasmosis and spiro- 

 chsetosis. and is a '' one-host " tick. The moults between larval 

 and nymphal stages and between the latter and the adult stage 

 are thus ])assed on the animal host, and because of this fact little 

 or no variation is met with in this portion of the life-cycle when 

 summer and winter seasons are com])ared. Tlie usual time 

 occupied in both seasons is about 23 days. The effects of 

 climatic conditions are, however, seen when we examine that 

 portion of the life-cycle between the time when the female drops 

 from the host and the time when the larvre emerges from the 

 eggs which she has laid, and to illustrate this point we may quote 

 from Fuller's and Theiler's observations in this respect. 

 The former says : 

 The female, after dropping from the host, starts laying in from five 

 days to two weeks in summer, while in winter at Capetown, at a tempera- 

 ture 65° to 70° F., she delays until the third or fourth week has passed. 

 Similarly the incubation period of the egg varies from three weeks to three 

 months. At a temperature of 85° to 90° in the incubator, hatched egRS 

 have been obtained in four weeks from the dropping of the mother tick, 

 which shows heat hastens development. Some moisture appears necessary 

 during the incubation period, as several batches of eggs kept in drv 

 boxes have shrivelled and failed to hatch. 



And again he says : 



The entire life cycle may be passed in two summer months, but, in 

 fact, probably seldom takes less than two and a half or three. There is 

 a possibility of three broods a year in our Cape climate, and a prob- 

 ability of at least two broods. 



In the climate of the Transvaal and in summer Theiler 

 has noted that the female usually begins to lay in five days after 

 dropping from the host, but in winter several weeks may elapse 



