164 INSECTS AND MAN 



of the fowl tick also exhibit another acquired habit : they 

 become engorged at night and rest by day. When fowls 

 are roosting, the ticks can take their fill of blood in peace ; 

 by day, their hosts are not above making a meal of such 

 unwelcome visitors. Other ticks again exhibit the pro- 

 tective habit of only attaching to inaccessible parts of their 

 host ; the usual feeding position of the spinous ear tick, 

 Ornithodorus megnini, within the external ear of cattle, 

 horses, etc., is a case in point. Certain species of the genus 

 Hcemaphysalis have, as hosts, quails and larks, but they 

 only engorge upon the birds' heads, where they are out of 

 danger. As a final example of protective habit, we may 

 mention that accelerated engorgement may be observed in 

 certain species of ticks ; in this way, the period of exposure 

 to enemies is much reduced. 



What, it may be asked, is the origin of tick-borne diseases ? 

 On the discovery of Texas fever in America in 1795, it 

 was noticed that when cattle from the south were brought 

 north, in summer, all the northern cattle along the route 

 sickened and died, whilst northern cattle taken south also 

 became diseased ; but the southern cattle, for the most part, 

 remained healthy, for they had been used to tick infection 

 from birth and had become immune, although those raised 

 in non-tick-infested districts were as susceptible to disease 

 as the northern animals. Similar diseases of cattle occur 

 in most parts of the world, but, in different countries, very 

 similar diseases are carried by very different ticks, a fact 

 which shows that the diseases have not originated in the 

 ticks. Probably all the species now carrying disease to 

 certain hosts were originally confined to other hosts, to 

 whom they were innocuous. Certain organisms were pro- 

 bably transferred by other ticks to other hosts, of the same 

 kind, without ill effect. But when a tick, after imbibing 

 the blood parasites of its natural host, became attached to 

 a new and different kind of host, then the blood parasites, 

 transferred to this alien blood, would quite probably origin- 



