INFLUENCE OF CLIMATE ON ANniAL DISEASE. 483. 



before she commences this operation. The eggs laid usually 

 hatch in three to six weeks, with an average of 36 days, in the 

 A\armer season, whilst in winter many weeks ma}' pass before 

 hatching occurs. He also remarks : 



The complete life-history requires, under tlit- uiost favouraMe condi- 

 tions, little more than two months. 



So much for the blue tick here, and now let us consider our^ 

 other examples. Both of these are " three-host " ticks, and 

 having thus to pass the periods between larva and nymph and 

 nymph and adult on the ground, it is ea.sy to see that the climatic 

 and tellurical conditions will affect their life-histories much more 

 than in the case of a " one-host '' tick such as the blue tick. 



Taking first the brown tick {R. appoidiadatus), which 

 is one of a number of species capable of transmitting East Coast 

 fever, and ]^robably the commonest agent in doing so, we shall 

 cjuote Theiler in regard to its life-history in the Transvaal ; but 

 it should be remarked that observations in regard to the effect 

 of climatic and local conditions on the development of this tick 

 have also been made in the Cape Province by Lounsbury. 

 Theiler, speaking of it, says : 



I-t is principally a summer tick, during- which time it is found on 

 ^^rious domesticated animals .... and prefers as its habitation 

 warm stretches of the country. It is abundant in the low veld, less fre- 

 quent in the middle veld, and is very rare, often entirely- absent, on the 

 plateau of the high veld, but it may be found there where in protected 

 vallej's the vegetation grows higher. 



The influence of climate on its development he has shown 

 in observations which may be condensed as follows. The female 

 tick, after dropping from the host, may commence to lay eggs 

 in six days, and in the summer these may hatch in an average 

 time of 28 days (the shortest period recorded being 13 days), 

 whilst in winter hatching may be delayed for several months. 

 The larvai now pass on to their host and there engorge, an 

 operation which may only occupy a period of three days. Hav- 

 ing done this, they drop to the ground to moult to the nymphal 

 stage, which moult occupies a period of 21 days on an average 

 in summer, whilst in winter it is increased in length. The 

 shortest period recognised in this part of the life-history was 16 

 days under the most favourable conditions. The nymph now 

 engorges in the same way, and in about the same time as the 

 larvae has done, and then again drops to moult, and in this case 

 the average time occupied was about 18 days. Theiler estiinates 

 that the whole life-cycle of this tick may thus^occupy about 73 

 days in summer, and under the most favourable conditioits, 

 whilst in winter he believes that it may take over six months for 

 its completion. 



The other tick of the *' three-host " group, namely, the bont 

 tick {Amblyomma hebrccum), has been taken to illustrate a point 

 referred to later on. This tick is, as Lounsbury has shown, the 

 agent responsible for the transmission of that disease of cattle, 

 sheep, and goats known popularly as heart-water. Speaking in 



