/( )()!.( KiY FOR MEDICAL STUDENTS CHAP. 



provided with recurved hooks, and these organs when thrust into the 

 tlu- host give a firm attachment. If the tick is forcibly pulled 

 utt. thc.M or-ans are left behind in the skin and may continue to cause 

 irritation for weeks. A tick should therefore be induced to detach itself 

 bv applying to its body a small drop of oil which will get into its stigmata 

 .ind iMiully cause it to release its hold. 



The majority of ticks are grouped together as the IXODIDAE : they 

 M-ni/.able by a stiff chitinous plate which covers in the male the 

 ureater part of the dorsal surface, and in the female the fore part, all the 

 hinder portion being enclosed in tough leathery skin. Ixodes is the 

 comnion slurp-tick. 1 Iloophilus and Rhipicephalus are of practical im- 

 portant e as transmitters of Piroplasma or Babesia (p. 64). 



Tlu- Arirasidae are without the stiff chitinous plate on the dorsal 



and the body bulges forwards so as to conceal the head region 



in a view from the dorsal side. Ornithodorus , common about native 



huts and ramping spots in tropical Africa,, is the transmitter of African 



Relapsing I- ever (p. 77). 



The Tarsonemidae are very minute mites which cause injury to 

 plants. A species of Tarsonenms has been found recently infesting the 

 trachea! tubes of Bees suffering from " Isle of Wight " disease, and is 

 believed by its discoverers to be the cause of the disease. 



The Trombidiidae include the red Harvest mites the larval stage of 

 whirh (" Harvest bug " ; Bete rouge ; Bicho Colorado) is apt to get on 

 to the skin of human beings in grassy country and causes intolerable 

 itching by its bite. 



V. The CRUSTACEA include a vast variety of arthropods adapted to 



an aquatic existence. In correlation with this the function of breathing 



is carried out by gills thin-walled projections of the body-surface spring- 



i rule from the appendages. And very commonly some of the 



append. iLM - arc, during at least part of the life-history, modified for 



swimming. The cuticle is usually stiffened by the deposit, within its 



substance, of salts of lime. The life-history commonly involves great 



in form between the larval and the adult phase, but these changes 



are brought about in successive steps without that re-organization of 



.illy the whole body that occurs in the metamorphosis of insects. 



^pinions feature which usually enables a crustacean to be recog- 



1 a -lance is afforded by the presence of two pairs of antennae. 



Th<- Cni>- ommonly classified in two main groups, the Mala- 



u.uue is sometimes erroneously applied to the " sheep-ked " 



