MITES AND TICKS 187 



the respiratory centres of the human or animal victim before the 

 tick has dropped off or been removed. 



The ill effects of ticks on their hosts have been recognised since 

 200 B.C. when M. Porcius Cato referred to treatments whereby 

 'there will be no sores and the wool will be more plentiful and in 

 better condition and the ticks (ricini) will not be troublesome'. 

 Earlier, Aristotle in his famous Historia Animalium made some 

 observations on the ecology of these ectoparasites and stated that 

 the 'tick is generated from couch grass'. He was also aware of the 

 harm that they caused. Despite the early realisation of the fact that 

 ticks are ectoparasitic on mammals, it was not until the late nine- 

 teenth century that their role in the transmission of disease was 

 first suspected. 



The common sheep tick is Ixodes reduviuSy found also on cattle 

 and horses. Its distribution in Britain has been shown to be in- 

 fluenced by superficial soil deposits. Where good pasture occurs 

 alongside 'islands' of damp ground, discrepancies in the distribu- 

 tion of the ticks are noted. Such 'islands' may harbour ticks while 

 the rest of the pasture is free, and when cattle move into these 

 'islands' they are infested (Arthur, 1952). Milne (1945) has shown 

 that the duration of the humidity level during daylight in conjunc- 

 tion with the temperature may decide the amount and extent of 

 activity of these animals in summer. 



During recent years considerable attention has been devoted to 

 the physiology and behaviour of ticks. For example. Lees (1947) 

 has shown that they owe their resistance to desiccation primarily to 

 a superficial layer of wax in the integument: after exposure to in- 

 creasing temperatures, water-loss increases abruptly at a certain 

 'critical temperature' as in insects. Species having higher critical 

 temperatures are more resistant to desiccation at temperatures 

 within the biological range. A broad correlation is possible between 

 these powers of resistance and the natural choice of habitat: Arga- 

 sidae infest dry, dusty situations, whereas Ixodidae occupy a wider 

 variety of ecological niches. Unfed ticks are able to take up water 

 rapidly through the wax-layer when exposed to high humidity. 

 This water uptake is dependent on the secretory activities of the 

 epidermal cells and is completely inhibited by abrasion of only a 

 part of the total cuticle surface which suggests that the cells are 



